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[CodeGenPrepare] Teach when it is profitable to speculate calls to @llvm.

Description

[CodeGenPrepare] Teach when it is profitable to speculate calls to @llvm.cttz/ctlz.

If the control flow is modelling an if-statement where the only instruction in
the 'then' basic block (excluding the terminator) is a call to cttz/ctlz,
CodeGenPrepare can try to speculate the cttz/ctlz call and simplify the control
flow graph.

Example:
\code
entry:

%cmp = icmp eq i64 %val, 0
br i1 %cmp, label %end.bb, label %then.bb

then.bb:

%c = tail call i64 @llvm.cttz.i64(i64 %val, i1 true)
br label %end.bb

end.bb:

%cond = phi i64 [ %c, %then.bb ], [ 64, %entry]

\code

In this example, basic block %then.bb is taken if value %val is not zero.
Also, the phi node in %end.bb would propagate the size-of in bits of %val
only if %val is equal to zero.

With this patch, CodeGenPrepare will try to hoist the call to cttz from %then.bb
into basic block %entry only if cttz is cheap to speculate for the target.

Added two new hooks in TargetLowering.h to let targets customize the behavior
(i.e. decide whether it is cheap or not to speculate calls to cttz/ctlz). The
two new methods are 'isCheapToSpeculateCtlz' and 'isCheapToSpeculateCttz'.
By default, both methods return 'false'.
On X86, method 'isCheapToSpeculateCtlz' returns true only if the target has
LZCNT. Method 'isCheapToSpeculateCttz' only returns true if the target has BMI.

Differential Revision: http://reviews.llvm.org/D6728