Do extremely simple sinking of instructions when they are only used in a

Do extremely simple sinking of instructions when they are only used in a
successor block.  This turns cases like this:

x = a op b
if (c) {
  use x
}

into:

if (c) {
  x = a op b
  use x
}

This triggers 3965 times in spec, and is tested by
Regression/Transforms/InstCombine/sink_instruction.ll

This appears to expose a bug in the X86 backend for 177.mesa, which I'm
looking in to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18677 91177308-0d34-0410-b5e6-96231b3b80d8