dag-combine optimization to implement the ext-load efficiently (using shuffles).
For example the type <4 x i8> is stored in memory as i32, but it needs to
find its way into a <4 x i32> register. Previously we scalarized the memory
access, now we use shuffles.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139995 91177308-0d34-0410-b5e6-96231b3b80d8
