Tighten check for vector types in fold_convertible_p (PR 92741)

Tighten check for vector types in fold_convertible_p (PR 92741)

In this PR, IPA-CP was misled into using NOP_EXPR rather than
VIEW_CONVERT_EXPR to reinterpret a vector of 4 shorts as a vector
of 2 ints.  This tripped the tree-cfg.c assert I'd added in r278245.

2019-12-02  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/92741
	* fold-const.c (fold_convertible_p): Check vector types more
	thoroughly.

gcc/testsuite/
	PR middle-end/92741
	* gcc.dg/pr92741.c: New test.

From-SVN: r278910