Add initial support for fixed-size local vardecls. This allows us to compile:

int func() {
  register int X;
  {
    int Y;

into:

define i32 @func() {
entry:
        %X = alloca i32         ; <i32*> [#uses=0]
        %Y = alloca i32         ; <i32*> [#uses=0]
        %allocapt = bitcast i32 undef to i32            ; <i32> [#uses=0]
...

llvm-svn: 39553