RISC-V: improve codegen for large constants with same 32-bit lo and hi parts [2]

RISC-V: improve codegen for large constants with same 32-bit lo and hi parts [2]

[part #2 of PR/109279]

SPEC2017 deepsjeng uses large constants which currently generates less than
ideal code. This fix improves codegen for large constants which have
same low and hi parts: e.g.

	long long f(void) { return 0x0101010101010101ull; }

Before
        li      a5,0x1010000
        addi    a5,a5,0x101
        mv      a0,a5
        slli    a5,a5,32
        add     a0,a5,a0
        ret

With patch
	li	a5,0x1010000
	addi	a5,a5,0x101
	slli	a0,a5,32
	add	a0,a0,a5
	ret

This is testsuite clean.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_split_integer): if loval is equal
	to hival, ASHIFT the corresponding regs.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>