RISC-V: Add else operand to masked loads [PR115336].

This patch adds else operands to masked loads.  Currently the default
else operand predicate just accepts "undefined" (i.e. SCRATCH) values.

	PR middle-end/115336
	PR middle-end/116059

gcc/ChangeLog:

	* config/riscv/autovec.md: Add else operand.
	* config/riscv/predicates.md (maskload_else_operand): New
	predicate.
	* config/riscv/riscv-v.cc (get_else_operand): Remove static.
	(expand_load_store): Use get_else_operand and adjust index.
	(expand_gather_scatter): Ditto.
	(expand_lanes_load_store): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr115336.c: New test.
	* gcc.target/riscv/rvv/autovec/pr116059.c: New test.