[RISCV] Don't allow vector types to be used with inline asm 'r' constraint

The 'r' constraint uses the GPR class. There is generic support
for bitcasting and extending/truncating non-integer VTs to the
required integer VT. This doesn't work for scalable vectors and
instead crashes.

To prevent this, explicitly reject vectors. Fixed vectors might
work without crashing, but it doesn't seem worthwhile to allow.

While there remove an unnecessary level of indentation in the
"vr" and "vm" constraint handling.

Differential Revision: https://reviews.llvm.org/D115810