[RISCV] Add rv64 run lines to rv32 MC layer tests for B extension

Remove common instructions from rv64 tests since they are now
covered by the rv64 run lines in the rv32 tests.

Add rv32-only* tests for a few cases that aren't common between
r32 and rv64.

Addresses review feedback from D95150.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D95272