match.pd: Fold logarithmic identities.

This patch implements 4 rules for logarithmic identities in match.pd
under -funsafe-math-optimizations:
1) logN(1.0/a) -> -logN(a). This avoids the division instruction.
2) logN(C/a) -> logN(C) - logN(a), where C is a real constant. Same as 1).
3) logN(a) + logN(b) -> logN(a*b). This reduces the number of calls to
log function.
4) logN(a) - logN(b) -> logN(a/b). Same as 4).
Tests were added for float, double, and long double.

The patch was bootstrapped and regtested on aarch64-linux-gnu and
x86_64-linux-gnu, no regression.
Additionally, SPEC 2017 fprate was run. While the transform does not seem
to be triggered, we also see no non-noise impact on performance.
OK for mainline?

Signed-off-by: Jennifer Schmitz <jschmitz@nvidia.com>

gcc/
	PR tree-optimization/116826
	PR tree-optimization/86710
	* match.pd: Fold logN(1.0/a) -> -logN(a),
	logN(C/a) -> logN(C) - logN(a),	logN(a) + logN(b) -> logN(a*b),
	and logN(a) - logN(b) -> logN(a/b).

gcc/testsuite/
	PR tree-optimization/116826
	PR tree-optimization/86710
	* gcc.dg/tree-ssa/log_ident.c: New test.

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