Implement a trivial optimization to reduce the number of compares emitted.

For:

  register short X;
  if (!X) {

We now produce:

        %tmp = load i16* %X             ; <i16> [#uses=1]
        %tobool = icmp ne i16 %tmp, 0           ; <i1> [#uses=1]
        %lnot = xor i1 %tobool, true            ; <i1> [#uses=1]
        br i1 %lnot, label %ifthen, label %ifend

instead of:

        %tmp = load i16* %X             ; <i16> [#uses=1]
        %tobool = icmp ne i16 %tmp, 0           ; <i1> [#uses=1]
        %lnot = xor i1 %tobool, true            ; <i1> [#uses=1]
        %lnot.ext = zext i1 %lnot to i32                ; <i32> [#uses=1]
        %tobool1 = icmp ne i32 %lnot.ext, 0             ; <i1> [#uses=1]
        br i1 %tobool1, label %ifthen, label %ifend

llvm-svn: 39560