[12/15][Clang][RISCV][NFC] Refine the way to check for Policy in riscv_vector_builtin_cg.inc

The current way creates a fallacy that checking for
`PolicyAttrs == TAIL_AGNOSTIC` is implicitly equivalant to
`TAIL_AGNOSTIC_MASK_UNDISTURBED`. This works under the assumption that
an unmasked intrinsic has a policy of TAMU. The expression here is
mis-leading and will not be correct when the default policy is not
TAMU.

As this patch-set targets to change the default policy from TAMU to
TAMA, this commit is necessary before changing the default.

This is the 12th 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

Differential Revision: https://reviews.llvm.org/D141789