Refine the notion of MayThrow in LICM to include a header specific version

In LICM, we have a check for an instruction which is guaranteed to execute and thus can't introduce any new faults if moved to the preheader. To handle a function which might unconditionally throw when first called, we check for any potentially throwing call in the loop and give up.

This is unfortunate when the potentially throwing condition is down a rare path. It prevents essentially all LICM of potentially faulting instructions where the faulting condition is checked outside the loop. It also greatly diminishes the utility of loop unswitching since control dependent instructions - which are now likely in the loops header block - will not be lifted by subsequent LICM runs.

define void @nothrow_header(i64 %x, i64 %y, i1 %cond) {
; CHECK-LABEL: nothrow_header
; CHECK-LABEL: entry
; CHECK: %div = udiv i64 %x, %y
; CHECK-LABEL: loop
; CHECK: call void @use(i64 %div)
entry:
  br label %loop
loop: ; preds = %entry, %for.inc
  %div = udiv i64 %x, %y
  br i1 %cond, label %loop-if, label %exit
loop-if:
  call void @use(i64 %div)
  br label %loop
exit:
  ret void
}

The current patch really only helps with non-memory instructions (i.e. divs, etc..) since the maythrow call down the rare path will be considered to alias an otherwise hoistable load.  The one exception is that it does kick in for loads which are known to be invariant without regard to other possible stores, i.e. those marked with either !invarant.load metadata of tbaa 'is constant memory' metadata.

Differential Revision: http://reviews.llvm.org/D6725