[DAGCombiner] Don't create illegal narrow stores

[DAGCombiner] Don't create illegal narrow stores

Narrowing stores when the target doesn't support the narrow version
forces the target to expand into a load-modify-store sequence, which
is highly suboptimal. The information narrowing throws away (legality
of the inverse transform) is hard to re-analyze. If the target doesn't
support a store of the narrow type, don't narrow even in pre-legalize
mode.

No test as this is DAGCombiner and depends on target bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370576 91177308-0d34-0410-b5e6-96231b3b80d8