Fix valgrind errors in vp8_sixtap_predict8x4_armv6().

This function was accessing values below the stack pointer, which
 can be corrupted by signal delivery at any time.

Change-Id: I92945b30817562eb0340f289e74c108da72aeaca

vp8/common/arm/armv6/sixtappredict8x4_v6.asm

@@ -25,10 +25,10 @@
 ;and the result is stored in transpose.
 |vp8_sixtap_predict8x4_armv6| PROC
     stmdb       sp!, {r4 - r11, lr}
-    sub         sp, sp, #184                ;reserve space on stack for temporary storage: 20x(8+1) +4
+    str         r3, [sp, #-184]!            ;reserve space on stack for temporary storage, store yoffset
 
     cmp         r2, #0                      ;skip first_pass filter if xoffset=0
-    str         r3, [sp], #4                ;store yoffset
+    add         lr, sp, #4                  ;point to temporary buffer
     beq         skip_firstpass_filter
 
 ;first-pass filter
@@ -45,7 +45,6 @@
     mov         r2, #0x90000                ; height=9 is top part of counter
 
     sub         r1, r1, #8
-    mov         lr, #20
 
 |first_pass_hloop_v6|
     ldrb        r6, [r0, #-5]               ; load source data
@@ -83,10 +82,10 @@
     tst         r2, #0xff                   ; test loop counter
     usat        r11, #8, r11, asr #7
     add         r12, r12, #0x40
-    strh        r11, [sp], lr               ; result is transposed and stored, which
+    strh        r11, [lr], #20              ; result is transposed and stored, which
     usat        r12, #8, r12, asr #7
 
-    strh        r12, [sp], lr
+    strh        r12, [lr], #20
 
     movne       r11, r6
     movne       r12, r7
@@ -107,8 +106,7 @@
 
     subs        r2, r2, #0x10000
 
-    mov         r6, #158
-    sub         sp, sp, r6
+    sub         lr, lr, #158
 
     add         r0, r0, r1                  ; move to next input line
 
@@ -116,10 +114,7 @@
 
 ;second pass filter
 secondpass_filter
-    mov         r1, #18
-    sub         sp, sp, r1                  ; 18+4
-
-    ldr         r3, [sp, #-4]               ; load back yoffset
+    ldr         r3, [sp], #4                ; load back yoffset
     ldr         r0, [sp, #216]              ; load dst address from stack 180+36
     ldr         r1, [sp, #220]              ; load dst stride from stack 180+40
 
@@ -192,30 +187,28 @@ skip_firstpass_filter
     sub         r0, r0, r1, lsl #1
     sub         r1, r1, #8
     mov         r2, #9
-    mov         r3, #20
 
 skip_firstpass_hloop
     ldrb        r4, [r0], #1                ; load data
     subs        r2, r2, #1
     ldrb        r5, [r0], #1
-    strh        r4, [sp], r3                ; store it to immediate buffer
+    strh        r4, [lr], #20               ; store it to immediate buffer
     ldrb        r6, [r0], #1                ; load data
-    strh        r5, [sp], r3
+    strh        r5, [lr], #20
     ldrb        r7, [r0], #1
-    strh        r6, [sp], r3
+    strh        r6, [lr], #20
     ldrb        r8, [r0], #1
-    strh        r7, [sp], r3
+    strh        r7, [lr], #20
     ldrb        r9, [r0], #1
-    strh        r8, [sp], r3
+    strh        r8, [lr], #20
     ldrb        r10, [r0], #1
-    strh        r9, [sp], r3
+    strh        r9, [lr], #20
     ldrb        r11, [r0], #1
-    strh        r10, [sp], r3
+    strh        r10, [lr], #20
     add         r0, r0, r1                  ; move to next input line
-    strh        r11, [sp], r3
+    strh        r11, [lr], #20
 
-    mov         r4, #158
-    sub         sp, sp, r4                  ; move over to next column
+    sub         lr, lr, #158                ; move over to next column
     bne         skip_firstpass_hloop
 
     b           secondpass_filter