[RISCV] Change the immediate argument to Zk* intrinsics/builtins from i8 to i32.

This matches gcc. It also lets us fix a bug that the byteselect
predicate was not being evaluated in tablegen. We can't have i8
TImmLeaf in tablegen because i8 isn't a type for any register class.

I've added AutoUpgrade support for the IR intrinsics.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D152627