[1/3][RISCV] Define machine instruction to write an immediate into vxrm

This patch-set wants to model rounding mode for the fixed-point
intrinsics of the RVV C intrinsics.

The specification PR: [riscv-non-isa/rvv-intrinsic-doc#222](https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/222)

The 3 patches is a proof-of-concept with a bottom-up approach
Going from machine instruction to LLVM intrinsics, then to the C
intrinsics. The 3 patches applies the rounding mode control on the
`vaadd` instruction. Proceeding patches will extend the change to all
other fixed-point computations.

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This is the 1st commit of the patch-set.  This patch gives a name to
the machine instruction that writes an immediate into the CSR `vxrm`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D151395