Add RTX codes for BITREVERSE and COPYSIGN.

An analysis of backend UNSPECs reveals that two of the most common UNSPECs
across target backends are for copysign and bit reversal.  This patch
adds RTX codes for these expressions to allow their representation to
be standardized, and them to optimized by the middle-end RTL optimizers.

2023-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* doc/rtl.texi (bitreverse, copysign): Document new RTX codes.
	* rtl.def (BITREVERSE, COPYSIGN): Define new RTX codes.
	* simplify-rtx.cc (simplify_unary_operation_1): Optimize
	NOT (BITREVERSE x) as BITREVERSE (NOT x).
	Optimize POPCOUNT (BITREVERSE x) as POPCOUNT x.
	Optimize PARITY (BITREVERSE x) as PARITY x.
	Optimize BITREVERSE (BITREVERSE x) as x.
	(simplify_const_unary_operation) <case BITREVERSE>: Evaluate
	BITREVERSE of a constant integer at compile-time.
	(simplify_binary_operation_1) <case COPYSIGN>:  Optimize
	COPY_SIGN (x, x) as x.  Optimize COPYSIGN (x, C) as ABS x
	or NEG (ABS x) for constant C.  Optimize COPYSIGN (ABS x, y)
	and COPYSIGN (NEG x, y) as COPYSIGN (x, y).
	Optimize COPYSIGN (x, ABS y) as ABS x.
	Optimize COPYSIGN (COPYSIGN (x, y), z) as COPYSIGN (x, z).
	Optimize COPYSIGN (x, COPYSIGN (y, z)) as COPYSIGN (x, z).
	(simplify_const_binary_operation): Evaluate COPYSIGN of constant
	arguments at compile-time.

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