[RISCV] Evaluate ICE operands to builtins using getIntegerConstantExpr.

Some RISC-V builtins requires ICE operands. We should call
getIntegerConstantExpr instead of EmitScalarExpr to match other
targets.

This was made a little trickier by the vector intrinsics not having
a valid type string, but there are two that have ICE operands so
I specified them manually.