[2/3][RISCV][POC] Model vxrm in LLVM intrinsics and machine instructions for RVV fixed-point instructions

Depends on D151395.

This is the 2nd patch of the patch-set. For the cover letter of the
patch-set, please checkout D151395. This patch originates from
D121376.

This commit models vxrm by adding an immediate operand into intrinsics
and machine instructions of RVV fixed-point instruction `vaadd`,
`vaaddu`, `vasub`, and `vasubu`. This commit only covers intrinsics of
the four instructions, the proceeding patches of the patch-set will do
the same to other RVV fixed-point instructions.

The current naiive approach is to have a write to vxrm inserted before
every fixed-point instruction. This is done by the new added pass
`RISCVInsertReadWriteCSR`. The reason to name the pass in a more general
term is because we will also model rounding mode for the RVV floating-
point instructions. The approach will be improved in the future,
implementing partial redundancy elimination algorithms to it.

The original LLVM intrinsics and machine instructions, take `vaadd` as
an example, does not model the rounding mode is not removed in this
patch. That is, `int.riscv.vaadd.*` co-exists with
`int.riscv.vaadd.rm.*` after this patch. The next patch will add C
intrinsics of vaadd with an additional operand that models the control
of the rounding mode, in this patch, `int.riscv.vaadd.rm.*` will
replace `int.riscv.vaadd.*`.

Authored-by: ShihPo Hung <shihpo.hung@sifive.com>
Co-Authored-by: eop Chen <eop.chen@sifive.com>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D151396

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