[DAGCombine] isLegalNarrowLoad function (NFC)

Pull the checks upon the load out from ReduceLoadWidth into their own
function.

Differential Revision: https://reviews.llvm.org/D40833


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319766 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed