So another constant synthesis improvement.

In this patch we're looking at cases where we'd like to be able to use
lui+slli, but can't because of the sign extending nature of lui on
TARGET_64BIT.  For example: 0x8001100000020UL.  The trunk currently generates 4
instructions for that constant, when it can be done with 3 (lui+slli.uw+addi).

When Zba is enabled, we can use lui+slli.uw as the slli.uw masks off the bits
32..63 before shifting, giving us the precise semantics we want.

I strongly suspect we'll want to do the same for a set of constants with
lui+add.uw, lui+shNadd.uw, so you'll see the beginnings of generalizing support
for lui followed by a "uw" instruction.

The new test just tests the set of cases that showed up while exploring a
particular space of the constant synthesis problem.  It's not meant to be
exhaustive (failure to use shadd when profitable).

gcc/

	* config/riscv/riscv.cc (riscv_integer_op): Add field tracking if we
	want to use a "uw" instruction variant.
	(riscv_build_integer_1): Initialize the new field in various places.
	Use lui+slli.uw for some constants.
	(riscv_move_integer): Handle slli.uw.

gcc/testsuite/

	* gcc.target/riscv/synthesis-2.c: New test.

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