[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.