[X86][X86FixupLEA] Rename processInstructionForSLM to processInstructionForSlowLEA (NFCI)

The function isn't SLM specific (its driven by the FeatureSlowLEA flag).

Minor tidyup prior to PR38225.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345836 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed