Extend neg_const_int simplifications to other const rtxes

Extend neg_const_int simplifications to other const rtxes

This patch generalises some neg_const_int-based rtx simplifications
so that they handle all CONST_SCALAR_INTs and also CONST_POLY_INT.
This actually simplifies things a bit, since we no longer have
to treat HOST_WIDE_INT_MIN specially.

This is tested by later SVE patches.

2019-09-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* simplify-rtx.c (neg_const_int): Replace with...
	(neg_poly_int_rtx): ...this new function.
	(simplify_binary_operation_1): Extend (minus x C) -> (plus X -C)
	to all CONST_SCALAR_INTs and to CONST_POLY_INT.
	(simplify_plus_minus): Likewise for constant terms here.

From-SVN: r276017