[FastISel] Fix patchpoint lowering to set the result register.

Always update the value map with the result register (if there is one), for the
patchpoint instruction we created to replace the target-specific call
instruction.
[FastISel] Insert patchpoint instruction before the target generated call instruction.

The patchpoint instruction should have been inserted before the target
generated call instruction to be inside the ADJSTACKDOWN/ADJSTACKUP call
sequence window.
[FastISel][X86] Implement the FastLowerCall hook.

This implements the FastLowerCall hook, which is based on the DoSelectCall
function. The implementation is very similar, but the target-independent call
lowering part has been factored out.

This should also enable patchpoint intrinsic lowering for FastISel on X86.

Related to <rdar://problem/17427052>.
[FastISel][X86] Implement the FastLowerIntrinsicCall hook.

Rename X86VisitIntrinsicCall -> FastLowerIntrinsicCall, which effectively
implements the target hook.
[FastISel][X86] Remove no longer needed functions.