[7/15][Clang][RISCV][NFC] Correct the default value for Policy to TAMU

The default value is set to `Omit`, but in fact the value is then
assigned in `updateNamesAndPolicy`, which is set to TUMU when masked and
TAMU when unmasked. This commit demonstrates so and further remove
another dependency of `Omit`.

This is the 7th 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: craig.topper, kito-cheng

Differential Revision: https://reviews.llvm.org/D141759