First tiny step to implementing PR9322: build infrastructure for only emitting the

First tiny step to implementing PR9322: build infrastructure for only emitting the
live case of a switch statement when switching on a constant.  This is terribly
limited, but enough to handle the trivial example included.  Before we would
emit:

define void @test1(i32 %i) nounwind {
entry:
  %i.addr = alloca i32, align 4
  store i32 %i, i32* %i.addr, align 4
  switch i32 1, label %sw.epilog [
    i32 1, label %sw.bb
  ]

sw.bb:                                            ; preds = %entry
  %tmp = load i32* %i.addr, align 4
  %inc = add nsw i32 %tmp, 1
  store i32 %inc, i32* %i.addr, align 4
  br label %sw.epilog

sw.epilog:                                        ; preds = %sw.bb, %entry
  switch i32 0, label %sw.epilog3 [
    i32 1, label %sw.bb1
  ]

sw.bb1:                                           ; preds = %sw.epilog
  %tmp2 = load i32* %i.addr, align 4
  %add = add nsw i32 %tmp2, 2
  store i32 %add, i32* %i.addr, align 4
  br label %sw.epilog3

sw.epilog3:                                       ; preds = %sw.bb1, %sw.epilog
  ret void
}

now we emit:

define void @test1(i32 %i) nounwind {
entry:
  %i.addr = alloca i32, align 4
  store i32 %i, i32* %i.addr, align 4
  %tmp = load i32* %i.addr, align 4
  %inc = add nsw i32 %tmp, 1
  store i32 %inc, i32* %i.addr, align 4
  ret void
}

This improves -O0 compile time (less IR to generate and shove through the code
generator) and the clever linux kernel people found a way to fail to build if we
don't do this optimization.  This step isn't enough to handle the kernel case
though.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126597 91177308-0d34-0410-b5e6-96231b3b80d8