Make get_value_for_expr check for INTEGER_CSTs

Make get_value_for_expr check for INTEGER_CSTs

CONSTANT lattice values are symbolic constants rather than
compile-time constants, so among other things can be POLY_INT_CSTs.
This patch fixes a case in which we assumed all CONSTANTs were either
ADDR_EXPRs or INTEGER_CSTs.

This is tested by later SVE patches.

2019-09-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-ssa-ccp.c (get_value_for_expr): Check whether CONSTANTs
	are INTEGER_CSTs.

From-SVN: r275871