Use ssizetype selectors for autovectorised VEC_PERM_EXPRs

Use ssizetype selectors for autovectorised VEC_PERM_EXPRs

The previous patches mean that there's no reason that constant
VEC_PERM_EXPRs need to have the same shape as the data inputs.
This patch makes the autovectoriser use sizetype elements instead,
so that indices don't get truncated for large or variable-length
vectors.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-cfg.c (verify_gimple_assign_ternary): Allow the size of
	the selector elements to be different from the data elements
	if the selector is a VECTOR_CST.
	* tree-vect-stmts.c (vect_gen_perm_mask_any): Use a vector of
	ssizetype for the selector.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256100 138bc75d-0d04-0410-961f-82ee72b054a4