Revert "[X86FixupLEAs] Sub register usage of LEA dest should block LEA/SUB optimization"

This reverts commit f35bcea1 because it
depends on 1b748faf, which breaks
building the llvm-test-suite with -verify-machineinstrs on X86.

See 154adc0f135cff3f8a8861c335d2b88c8049d098 for more details.