[RISCV] Use AdjustInstrPostInstrSelection to insert a FRM dependency for scalar FP...

[RISCV] Use AdjustInstrPostInstrSelection to insert a FRM dependency for scalar FP instructions with dynamic rounding mode.

In order to support constrained FP intrinsics we need to model FRM
dependency. Whether or not a instruction uses FRM is based on a 3
bit field in the instruction. Because of this we can't add
'Uses = [FRM]' to the tablegen descriptions.

This patch examines the immediate after isel and adds an implicit
use of FRM. This idea came from Roger Ferrer Ibanez.

Other ideas:
We could be overly conservative and just pretend all instructions with
frm field read the FRM register. Or we could have pseudoinstructions
for CodeGen with rounding mode.

Reviewed By: asb, frasercrmck, arcbbb

Differential Revision: https://reviews.llvm.org/D115555