[DAGCombine] Enable more srl -> load combines

Change the calculation for the desired ValueType for non-sign
extending loads, as in those cases we don't care about the
higher bits. This creates a smaller ExtVT and allows for such
combinations as:
(srl (zextload i16, [addr]), 8) -> (zextload i8, [addr + 1])

Differential Revision: https://reviews.llvm.org/D40034

llvm-svn: 318390