GlobalISel: Handle llvm.read_register

SelectionDAG has a bunch of machinery to defer this to selection time
for some reason. Just directly emit a copy during IRTranslator. The
x86 usage does somewhat questionably check hasFP, which could depend
on the whole function being at minimum translated.

This does lose the convergent bit if the callsite had it, which may be
a problem. We also lose that in general for intrinsics, which may also
be a problem.

llvm-svn: 373294