[to-be-committed,RISC-V] Improve usage of slli.uw in constant synthesis

[to-be-committed,RISC-V] Improve usage of slli.uw in constant synthesis

And an improvement to using slli.uw...

I recently added the ability to use slli.uw in the synthesis path.  That
code was conditional on the right justified constant being a LUI_OPERAND
after sign extending from bit 31 to bit 63.

That code is working fine, but could be improved.  Specifically there's
no reason it shouldn't work for LUI+ADDI under the same circumstances.
So rather than testing the sign extended, right justified constant is a
LUI_OPERAND, we can just test that the right justified constant has
precisely 32 leading zeros.

gcc/
	* config/riscv/riscv.cc (riscv_build_integer_1): Use slli.uw more.

gcc/testsuite
	* gcc.target/riscv/synthesis-5.c: New test.