[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176052 91177308-0d34-0410-b5e6-96231b3b80d8