simplify-rtx: Implement constant folding of SS_TRUNCATE, US_TRUNCATE

simplify-rtx: Implement constant folding of SS_TRUNCATE, US_TRUNCATE

This patch implements RTL constant-folding for the SS_TRUNCATE and US_TRUNCATE codes.
The semantics are a clamping operation on the argument with the min and max of the narrow mode,
followed by a truncation. The signedness of the clamp and the min/max extrema is derived from
the signedness of the saturating operation.

We have a number of instructions in aarch64 that use SS_TRUNCATE and US_TRUNCATE to represent
their operations and we have pretty thorough runtime tests in gcc.target/aarch64/advsimd-intrinsics/vqmovn*.c.
With this patch the instructions are folded away at optimisation levels and the correctness checks still
pass.

Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.

gcc/ChangeLog:

	* simplify-rtx.cc (simplify_const_unary_operation):
	Handle US_TRUNCATE, SS_TRUNCATE.