X86: Teach X86InstrInfo::analyzeCompare to recognize compares of symbols.

This requires that we communicate to X86InstrInfo::optimizeCompareInstr
that the second operand is neither a register nor an immediate. The way we
do that is by setting CmpMask to zero.

Note that there were already instructions where the second operand was not a
register nor an immediate, namely X86::SUB*rm, so also set CmpMask to zero
for those instructions. This seems like a latent bug, but I was unable to
trigger it.

Differential Revision: https://reviews.llvm.org/D28621

llvm-svn: 294634