Implement scaffolding for lvalues. Implement block vardecl lvalues.

This allows us to translate:

int func() {
  register int X;
  {
    int Y;
    return 1+X+Y;
  }
}

into:

define i32 @func() {
entry:
        %X = alloca i32         ; <i32*> [#uses=1]
        %Y = alloca i32         ; <i32*> [#uses=1]
        %allocapt = bitcast i32 undef to i32            ; <i32> [#uses=0]
        %tmp = load i32* %X             ; <i32> [#uses=1]
        %tmp1 = add i32 1, %tmp         ; <i32> [#uses=1]
        %tmp2 = load i32* %Y            ; <i32> [#uses=1]
        %tmp3 = add i32 %tmp1, %tmp2            ; <i32> [#uses=1]
        ret i32 %tmp3
                ; No predecessors!
        ret i32 undef
}

llvm-svn: 39555