[IR] Update llvm.prefetch to match docs

The current llvm.prefetch intrinsic docs state "The rw, locality and
cache type arguments must be constant integers."

This change:
- Makes arg 3 (cache type) an ImmArg
- Improves the verifier error messages to reference the incorrect
  argument.
- Fixes two tests which contradict the docs.

This is needed as the lowering to GlobalISel is different for ImmArgs
compared to other constants. The non-ImmArgs create a G_CONSTANT MIR
instruction, the for ImmArgs the constant is put directly on the
intrinsic's MIR instruction as an immediate.

Differential Revision: https://reviews.llvm.org/D132042