[X86] Merge X86MCInstLowering's maxLongNopLength into emitNop and remove check for FeatureNOPL.

The switch in emitNop uses 64-bit registers for nops exceeding
2 bytes. This isn't valid outside 64-bit mode. We could fix this
easily enough, but there are no users that ask for more than 2
bytes outside 64-bit mode.

Inlining the method to make the coupling between the two methods
more explicit.