RISC-V: Optimize switch with sign-extended index.

RISC-V: Optimize switch with sign-extended index.

	gcc/
	* expr.c (do_tablejump): When converting index to Pmode, if we have a
	sign extended promoted subreg, and the range does not have the sign bit
	set, then do a sign extend.

	* config/riscv/riscv.c (riscv_extend_comparands): In unsigned QImode
	test, check for sign extended subreg and/or constant operands, and
	do a sign extend in that case.

	gcc/testsuite/
	* gcc.target/riscv/switch-qi.c: New.
	* gcc.target/riscv/switch-si.c: New.

From-SVN: r260340