[x86] fix allowsMisalignedMemoryAccess() implementation

This patch fixes the x86 implementation of allowsMisalignedMemoryAccess() to correctly
return the 'Fast' output parameter for 32-byte accesses. To test that, an existing load
merging optimization is changed to use the TLI hook. This exposes a shortcoming in the
current logic and results in the regression test update. Changing other direct users of
the isUnalignedMem32Slow() x86 CPU attribute would be a follow-on patch.

Without the fix in allowsMisalignedMemoryAccesses(), we will infinite loop when targeting
SandyBridge because LowerINSERT_SUBVECTOR() creates 32-byte loads from two 16-byte loads
while PerformLOADCombine() splits them back into 16-byte loads.

Differential Revision: http://reviews.llvm.org/D10662

llvm-svn: 245075