[x64/ia32] Deal with the non-transitivity of InstructionSelector::CanCover() when folding loads into branches.

Sequences like:

1: Load[kRepWord32|kTypeInt32](<address>, ...)
2: Word32And(1, <constant>)
3: Word32Equal(2, <another constant>)
4: Store[(kRepWord32 : NoWriteBarrier)](<address>, <value>)
5: Branch[None](3, ...) -> B1, B2

where #1 and #4 refer to the same memory location, are problematic because in VisitBranch we assume that 'InstructionSelector::CanCover()' is transitive.

What happens is that CanCover(5, 3) is true (3 is a pure op), and so are CanCover(3, 2), CanCover(2, 1), but the effect level of 5 and 3 never gets checked because 3 is a pure op. Upon VisitBranch, we ended up materializing:

mov [address], <value>
test [address], <another constant>

With this patch, it becomes:

mov reg, [address]
mov [address], <value>
test reg, <another constant>

BUG=chromium:611976

Review-Url: https://codereview.chromium.org/2008493002
Cr-Commit-Position: refs/heads/master@{#36482}