Add a if-conversion optimization that allows 'true' side of a diamond to be

unpredicated. That is, turn
 subeq  r0, r1, #1
 addne  r0, r1, #1                                                                                                                                                                                                     
into
 sub    r0, r1, #1
 addne  r0, r1, #1

For targets where conditional instructions are always executed, this may be
beneficial. It may remove pseudo anti-dependency in out-of-order execution
CPUs. e.g.
 op    r1, ...
 str   r1, [r10]        ; end-of-life of r1 as div result
 cmp   r0, #65
 movne r1, #44  ; raw dependency on previous r1
 moveq r1, #12

If movne is unpredicated, then
 op    r1, ...
 str   r1, [r10]
 cmp   r0, #65
 mov   r1, #44  ; r1 written unconditionally
 moveq r1, #12

Both mov and moveq are no longer depdendent on the first instruction. This gives
the out-of-order execution engine more freedom to reorder them.

This has passed entire LLVM test suite. But it has not been enabled for any ARM
variant pending more performance evaluation.

rdar://8951196

llvm-svn: 146914