Check for bitwise identity when encoding VECTOR_CSTs (PR 92768)

Check for bitwise identity when encoding VECTOR_CSTs (PR 92768)

This PR shows that we weren't checking for bitwise-identical values
when trying to encode a VECTOR_CST, so -0.0 was treated the same as
0.0 for -fno-signed-zeros.  The patch adds a new OEP flag to select
that behaviour.

2019-12-05  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/92768
	* tree-core.h (OEP_BITWISE): New flag.
	* fold-const.c (operand_compare::operand_equal_p): Handle it.
	* tree-vector-builder.h (tree_vector_builder::equal_p): Pass it.

gcc/testsuite/
	PR middle-end/92768
	* gcc.dg/pr92768.c: New test.

From-SVN: r279002