[X86FixupLEAs] Hoist the calls to isLEA out of the 3 separate functions and...

[X86FixupLEAs] Hoist the calls to isLEA out of the 3 separate functions and put it in the basic block instruction loop. NFC

Now need to check it 3 different times. Just do it once at the top of the loop.

llvm-svn: 359658