RISC-V: Improve bitwise and ashift reassociation for single-bit immediate without zbs [PR 115921]

When zbs is not available, there's nothing special with single-bit
immediates and we should perform reassociation as normal immediates.

gcc/ChangeLog:

	PR target/115921
	* config/riscv/riscv.md (<optab>_shift_reverse): Only check
	popcount_hwi if !TARGET_ZBS.