[RISCV] Don't set the SplatOperand flag on intrinsics that take a shift amount.

The shift amount should always be a vector or an XLen scalar.
The SplatOperand flag is used to indicate we need to legalize
non-XLen scalars including special handling for i64 on RV32.
This will prevent us from silently adjusting these operands if
the intrinsics are misused.

I'll probably adjust the name of the SplatOperand flag slightly
in a follow up patch.

Reviewed By: khchen, frasercrmck

Differential Revision: https://reviews.llvm.org/D99545