[RISCV] Fix PR 106632 and PR 106588 a few constraints in bitmanip.md

[RISCV] Fix PR 106632 and PR 106588 a few constraints in bitmanip.md

The constraints should be n instead of i. Also there
needs to a check for out of bounds zero_extract for
*bexti.

gcc/ChangeLog:

	PR target/106632
	PR target/106588
	* config/riscv/bitmanip.md (*shNadduw): Use n constraint
	instead of i.
	(*slliuw): Likewise.
	(*bexti): Likewise. Also add a check for operands[2] to be less
	than the mode bitsize.