bswap: Fix up recent vector CONSTRUCTOR optimization [PR98568]

bswap: Fix up recent vector CONSTRUCTOR optimization [PR98568]

As the testcase shows, bswap can match even byte-swapping or indentity
from low part of some wider SSA_NAME.
For bswap replacement other than for vector CONSTRUCTOR the code has been
using NOP_EXPR casts if the types weren't compatible, but for vectors
we need to use VIEW_CONVERT_EXPR.  The problem with the latter is that
we require that it has the same size, which isn't guaranteed, so this patch
in those cases first adds a narrowing NOP_EXPR cast and only afterwards
does a VIEW_CONVERT_EXPR.

2021-01-07  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98568
	* gimple-ssa-store-merging.c (bswap_view_convert): New function.
	(bswap_replace): Use it.

	* g++.dg/torture/pr98568.C: New test.