Bug 1530844 - Make push/pop pair for ecx conditional on previous ecx content r=mgaudet

The ECX register must contain the SHIFT amount. Before this change, ECX will always be push/pop during a SHIFT operation. However, ECX should only be push/pop if it's original contents are going to be overwritten. We now only push/pop ECX if it contains some arbituary value and hence will be overwritten by the SHIFT operation.

Differential Revision: https://phabricator.services.mozilla.com/D22887

--HG--
extra : moz-landing-system : lando

js/src/jit/x86-shared/MacroAssembler-x86-shared-inl.h

@@ -328,7 +328,10 @@ inline void FlexibleShift32(MacroAssembler& masm, Register shift,
   // Add registers we may clobber and want to ensure are restored as live, and
   // remove what we definitely clobber (the destination)
   LiveRegisterSet preserve;
+
+  if (shift != ecx) {
     preserve.add(ecx);
+  }
   preserve.add(internalSrcDest);
 
   preserve.takeUnchecked(srcDest);