RISC-V: Remove gather scale and offset handling.

RISC-V: Remove gather scale and offset handling.

With the recent vectorizer changes upstream the vectorizer can take care
of offset extension and scaling (and its proper costing) itself.
Thus, we can remove all related handling in expand_gather_scatter and
set the predicates in the gather/scatter expanders to what our
instructions actually support.

gcc/ChangeLog:

	* config/riscv/autovec.md: Use const_1_operand for scale and
	extend predicates.
	* config/riscv/riscv-v.cc (expand_gather_scatter): Remove scale
	and extension handling.