X87: [x64/ia32] Deal with the non-transitivity of InstructionSelector::CanCover() when folding loads into branches.

    port 0d22e7e4 (r36482)

    original commit message:
    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

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

    With this patch, it becomes:

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

BUG=

Review-Url: https://codereview.chromium.org/2006223004
Cr-Commit-Position: refs/heads/master@{#36501}