[RISCV] Use bitsLE instead of strict == MVT::i32 in assertsexti32 and assertzexti32.

The patterns that use this really want to know if the operand has at
least 32 sign/zero bits.

This increases opportunities to use W instructions when the original
source used i8/i16. Not sure how much this matters for performance,
but it makes i8/i16 code more consistent with i32.