[RISCV] Check that the stack adjust immediate for cm.push/pop* has the correct...

[RISCV] Check that the stack adjust immediate for cm.push/pop* has the correct sign and is divisible by 16. (#85295)

To do this I've added a new AsmOperand for cm.push to expect a negative
value. We also use that to customize the print function so that we don't
need to detect cm.push opcode to add the negative sign.

I've renamed some places that used Spimm to be StackAdj since that's
what is being parsed. I'm still not about where we should use Spimm or
StackAdj.

I've removed the printSpimm helper function which in one usage printed
the sp[5:4]<<4 value and the other usage printed the full stack
adjustment. There wasn't anything interesting about how it was printed
it just passed the value to the raw_stream. If there was something
special needed, it's unclear whether it would be the same for the two
different usages so I inlined it.

One open question is whether we need to support stack adjustments
expressed as an expression rather than a literal integer.