[RISCV] Peephole optimization to fold merge.vvm and unmasked intrinsics.

The patch uses peephole method to fold merge.vvm and unmasked intrinsics to
masked intrinsics. Using peephole intead of tablegen patterns is to avoid large
auto gnerated code.

Note: The patch ignores segment loads since I don't know how to test them.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D130442