[9/15][Clang][RISCV][NFC] Use correct type for `RVVTypeCache::computeTypes` under RISCVVEmitter.cpp

`MaskedPrototype` is initialized and used nowhere, this is a bug. The
existing codebase works correctly upon this bug because the default
policy for unmasked intrinsics is set to TAMU.

This is something to be fixed because when the default policy is
changed to TAMA, clang will generate incorrect result.

This is the 9th 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/D141764