[PATCH 1/2 v2] RISC-V: Additional large constant synthesis improvements

[PATCH 1/2 v2] RISC-V: Additional large constant synthesis improvements

Changes since v1:
	- Fix bit31.
	- Remove negative shift checks.
	- Fix synthesis-7.c expected output.

-- >8 --

Improve handling of large constants in riscv_build_integer, generate
better code for constants where the high half can be constructed
by shifting/shiftNadding the low half or if the halves differ by less
than 2k.

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_build_integer): Detect new case
	of constants that can be improved.
	(riscv_move_integer): Add synthesys for concatening constants
	without Zbkb.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/synthesis-7.c: Adjust expected output.
	* gcc.target/riscv/synthesis-12.c: New test.
	* gcc.target/riscv/synthesis-13.c: New test.
	* gcc.target/riscv/synthesis-14.c: New test.