[CGP] despeculate expensive cttz/ctlz intrinsics

[CGP] despeculate expensive cttz/ctlz intrinsics

This is another step towards allowing SimplifyCFG to speculate harder, but then have
CGP clean things up if the target doesn't like it.

Previous patches in this series:
http://reviews.llvm.org/D12882
http://reviews.llvm.org/D13297

D13297 should catch most expensive ops, but speculation of cttz/ctlz requires special
handling because of weirdness in the intrinsic definition for handling a zero input
(that definition can probably be blamed on x86).

For example, if we have the usual speculated-by-select expensive op pattern like this:

  %tobool = icmp eq i64 %A, 0
  %0 = tail call i64 @llvm.cttz.i64(i64 %A, i1 true)   ; is_zero_undef == true
  %cond = select i1 %tobool, i64 64, i64 %0
  ret i64 %cond

There's an instcombine that will turn it into:

  %0 = tail call i64 @llvm.cttz.i64(i64 %A, i1 false)   ; is_zero_undef == false

This CGP patch is looking for that case and despeculating it back into:

  entry:
    %tobool = icmp eq i64 %A, 0
    br i1 %tobool, label %cond.end, label %cond.true

  cond.true:
    %0 = tail call i64 @llvm.cttz.i64(i64 %A, i1 true)    ; is_zero_undef == true
    br label %cond.end

  cond.end:
    %cond = phi i64 [ %0, %cond.true ], [ 64, %entry ]
    ret i64 %cond

This unfortunately may lead to poorer codegen (see the changes in the existing x86 test),
but if we increase speculation in SimplifyCFG (the next step in this patch series), then
we should avoid those kinds of cases in the first place.

The need for this patch was originally mentioned here:
http://reviews.llvm.org/D7506
with follow-up here:
http://reviews.llvm.org/D7554

Differential Revision: http://reviews.llvm.org/D14630

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253573 91177308-0d34-0410-b5e6-96231b3b80d8