[fast-isel] Add X86FastIsel::FastLowerArguments to handle functions with 6 or

fewer scalar integer (i32 or i64) arguments. It completely eliminates the need
for SDISel for trivial functions.

Also, add the new llc -fast-isel-abort-args option, which is similar to
-fast-isel-abort option, but for formal argument lowering.

llvm-svn: 176052