[8/15][Clang][RISCV][NFC] Always emit PolicyAttr in riscv_vector_builtin_cg.inc

The extra logic here is redundant. Also sneaked in an one-liner for
emitting `IsMasked`.

This is the 8th 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/D141762