This tries to allow libcalls to be tail called, using a similar method
to DAG where the type is checked to make sure they match, and if so the
backend, through lowerCall checks that the tailcall is valid for all
arguments.
The change implements support of the intrinsics `get_fpmode`,
`set_fpmode` and `reset_fpmode` in Global Instruction Selector. Now they
are lowered into library function calls.
Differential Revision: https://reviews.llvm.org/D158260