[10/15][Clang][RISCV][NFC] Don't need to check for `MaskPolicy` in `isTAPolicy` and `isTUPolicy`

Caller of the two utilities is always companied with a predicate to
check for `!IsMask`, so we don't need to check for the mask policy
here.

This also removes dependency to `Policy::PolicyType::Omit`, which will
be removed.

This is the 10th commit of a patch-set that aims to change the default policy
for RVV intrinsics from TAMU to TAMA.

Please refer to the cover letter in the 1st commit (D141573) for an
overview.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D141767