reassoc: Optimize in reassoc x < 0 && y < 0 to (x | y) < 0 etc. [PR95731]

We already had x != 0 && y != 0 to (x | y) != 0 and
x != -1 && y != -1 to (x & y) != -1 and
x < 32U && y < 32U to (x | y) < 32U, this patch adds signed
x < 0 && y < 0 to (x | y) < 0.  In that case, the low/high seem to be
always the same and just in_p indices whether it is >= 0 or < 0,
also, all types in the same bucket (same precision) should be type
compatible, but we can have some >= 0 and some < 0 comparison mixed,
so the patch handles that by using the right BIT_IOR_EXPR or BIT_AND_EXPR
and doing one set of < 0 or >= 0 first, then BIT_NOT_EXPR and then the other
one.  I had to move optimize_range_tests_var_bound before this optimization
because that one deals with signed a >= 0 && a < b, and limited it to the
last reassoc pass as reassoc itself can't virtually undo this optimization
yet (and not sure if vrp would be able to).

2021-01-12  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/95731
	* tree-ssa-reassoc.c (optimize_range_tests_cmp_bitwise): Also optimize
	x < 0 && y < 0 && z < 0 into (x | y | z) < 0 for signed x, y, z.
	(optimize_range_tests): Call optimize_range_tests_cmp_bitwise
	only after optimize_range_tests_var_bound.

	* gcc.dg/tree-ssa/pr95731.c: New test.
	* gcc.c-torture/execute/pr95731.c: New test.

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