[FastISel] Provide the option to skip target-independent instruction selection. NFC.

This allows the target to disable target-independent instruction selection and
jump directly into the target-dependent instruction selection code.

This can be beneficial for targets, such as AArch64, which could emit much
better code, but never got a chance to do so, because the target-independent
instruction selector was able to find an instruction sequence.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216947 91177308-0d34-0410-b5e6-96231b3b80d8