[PATCH 2/2 v2] RISC-V: Constant synthesis of inverted halves

Changes since v1:
	- Fix synthesis-15.c.

-- >8 --

Improve handling of constants where the high half can be constructed by
inverting the lower half.

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_build_integer): Detect constants
	were the higher half is the lower half inverted.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/synthesis-15.c: New test.