[X86] Use MOVSX by default instead of CBW to extend i8 to AX for i8 sdivrem.

[X86] Use MOVSX by default instead of CBW to extend i8 to AX for i8 sdivrem.

We can use a MOVSX16 here then rely on FixupBWInst to change to
MOVSX32 if the upper bits are dead. With a special case to
not promote if it could be turned into CBW.

Then we can rely on X86MCInstLower to turn the MOVSX into CBW
very late if register allocation worked out.

Using MOVSX gives an opportunity to use the MOVSX as a both a
copy and a sign extend since the input and output register aren't
tied together.

Differential Revision: https://reviews.llvm.org/D67192

llvm-svn: 371243