Fix over-widening handling of COND_EXPRs (PR 86749)

Fix over-widening handling of COND_EXPRs (PR 86749)

This PR is a wrong-code bug caused by the over-widening support.
The minimum input precisions for a COND_EXPR are supposed to apply
only to the "then" and "else" values, but here we were applying
them to the operands of a nested COND_EXPR comparison instead.

2018-08-01  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86749
	* tree-vect-patterns.c (vect_determine_min_output_precision_1):
	If the lhs is used in a COND_EXPR, check that it is being used
	as the "then" or "else" value.

gcc/testsuite/
	PR tree-optimization/86749
	* gcc.dg/vect/pr86749.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263213 138bc75d-0d04-0410-961f-82ee72b054a4