Expand unaligned 32 bit loads from an address which is a constant

Expand unaligned 32 bit loads from an address which is a constant
offset from a 32 bit aligned base as follows:

  ldw low, base[offset >> 2]
  ldw high, base[(offset >> 2) + 1]
  shr low_shifted, low, (offset & 0x3) * 8
  shl high_shifted, high, 32 - (offset & 0x3) * 8
  or result, low_shifted, high_shifted

Expand 32 bit loads / stores with 16 bit alignment into two 16 bit
loads / stores.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75902 91177308-0d34-0410-b5e6-96231b3b80d8