Fix case when Phi variable is a loop variable.

Need to flush variable to static expression rather than a variable.

reference/shaders/asm/frag/phi-loop-variable.asm.frag

@@ -0,0 +1,9 @@
+#version 450
+
+void main()
+{
+    for (int _22 = 35; _22 >= 0; _22--)
+    {
+    }
+}
+

shaders/asm/frag/phi-loop-variable.asm.frag

@@ -0,0 +1,71 @@
+; SPIR-V
+; Version: 1.0
+; Generator: Khronos Glslang Reference Front End; 1
+; Bound: 59
+; Schema: 0
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %v2float = OpTypeVector %float 2
+%mat2v2float = OpTypeMatrix %v2float 2
+%_ptr_Function_mat2v2float = OpTypePointer Function %mat2v2float
+    %v3float = OpTypeVector %float 3
+         %11 = OpTypeFunction %v3float %_ptr_Function_mat2v2float
+%_ptr_Function_v3float = OpTypePointer Function %v3float
+    %float_1 = OpConstant %float 1
+         %18 = OpConstantComposite %v3float %float_1 %float_1 %float_1
+        %int = OpTypeInt 32 1
+%_ptr_Function_int = OpTypePointer Function %int
+     %int_35 = OpConstant %int 35
+      %int_0 = OpConstant %int 0
+       %bool = OpTypeBool
+      %int_1 = OpConstant %int 1
+          %4 = OpFunction %void None %3
+          %5 = OpLabel
+               OpBranch %48
+         %48 = OpLabel
+         %58 = OpPhi %int %int_35 %5 %56 %50
+               OpLoopMerge %49 %50 None
+               OpBranch %51
+         %51 = OpLabel
+         %53 = OpSGreaterThanEqual %bool %58 %int_0
+               OpBranchConditional %53 %54 %49
+         %54 = OpLabel
+               OpBranch %50
+         %50 = OpLabel
+         %56 = OpISub %int %58 %int_1
+               OpBranch %48
+         %49 = OpLabel
+               OpReturn
+               OpFunctionEnd
+         %13 = OpFunction %v3float None %11
+         %12 = OpFunctionParameter %_ptr_Function_mat2v2float
+         %14 = OpLabel
+         %16 = OpVariable %_ptr_Function_v3float Function
+         %21 = OpVariable %_ptr_Function_int Function
+               OpStore %16 %18
+               OpStore %21 %int_35
+               OpBranch %23
+         %23 = OpLabel
+               OpLoopMerge %25 %26 None
+               OpBranch %27
+         %27 = OpLabel
+         %28 = OpLoad %int %21
+         %31 = OpSGreaterThanEqual %bool %28 %int_0
+               OpBranchConditional %31 %24 %25
+         %24 = OpLabel
+               OpBranch %26
+         %26 = OpLabel
+         %32 = OpLoad %int %21
+         %34 = OpISub %int %32 %int_1
+               OpStore %21 %34
+               OpBranch %23
+         %25 = OpLabel
+         %35 = OpLoad %v3float %16
+               OpReturnValue %35
+               OpFunctionEnd

spirv_glsl.cpp

@@ -6815,8 +6815,27 @@ void CompilerGLSL::flush_phi(uint32_t from, uint32_t to)
 	auto &child = get<SPIRBlock>(to);
 
 	for (auto &phi : child.phi_variables)
+	{
 		if (phi.parent == from)
-			statement(to_expression(phi.function_variable), " = ", to_expression(phi.local_variable), ";");
+		{
+			auto &var = get<SPIRVariable>(phi.function_variable);
+
+			// A Phi variable might be a loop variable, so flush to static expression.
+			if (var.loop_variable && !var.loop_variable_enable)
+				var.static_expression = phi.local_variable;
+			else
+			{
+				flush_variable_declaration(phi.function_variable);
+
+				// This might be called in continue block, so make sure we
+				// this to emit ESSL 1.0 compliant increments/decrements.
+				auto lhs = to_expression(phi.function_variable);
+				auto rhs = to_expression(phi.local_variable);
+				if (!optimize_read_modify_write(lhs, rhs))
+					statement(lhs, " = ", rhs, ";");
+			}
+		}
+	}
 }
 
 void CompilerGLSL::branch(uint32_t from, uint32_t to)