[PowerPC] Enable fast-isel on AIX 64 subtarget

This patch basically enables fast-isel for AIX 64-bit subtarget
(previously enabled only for ELF 64). The initial motivation is to
introduce branch folding to AIX generated code for correct debug
behavior. I also saw some compiling time improvement in a few LLVM
test-suite benchmarks. (toast, dbms, cjpeg, burg, etc.)

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D98844