[RISCV] Improve use of BSETI/BCLRI in constant materialization. (#91546)

We failed to use BSETI when bit 31 was set and a few bits above bit 31
were set. We also failed to use multiple BSETI when the low 32 bits were
zero.

I've removed the special cases for constants 0x80000000-0xffffffff and
wrote a more generic algorithm for BSETI.

I've rewritten the BCLRI handling to be similar to the new BSETI
algorithm. This picks up cases where bit 31 is 0 and only a few high
bits are 0.