Implement Math.log2 via ported extract from fdlibm.

Adapted from Raymond Toy's (rtoy@chromium.org) port, extracted from fdlibm's pow implementation.

R=rtoy@chromium.org
BUG=v8:3579
LOG=N

Review URL: https://codereview.chromium.org/786823003

Cr-Commit-Position: refs/heads/master@{#25768}
Disable invalid DCHECK_EQ in serializer.cc

BUG=none
R=mvstanton@chromium.org,yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/791363002

Cr-Commit-Position: refs/heads/master@{#25769}
remove Isolate::debugger_initialized_

It's never used.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/797543002

Cr-Commit-Position: refs/heads/master@{#25770}
[turbofan]: Port lea changes to ia32

Review URL: https://codereview.chromium.org/747283005

Cr-Commit-Position: refs/heads/master@{#25771}
[turbofan] commit allocated registers early

R=bmeurer@chromium.org
BUG=441107
LOG=N

Review URL: https://codereview.chromium.org/795043003

Cr-Commit-Position: refs/heads/master@{#25772}
[turbofan] Mark arm64 cbz/cbnz tbz/tbnz instructions as branch instructions.

The instruction selector now selects pseudo instructions: CompareAndBranch or
TestAndBranch which are associated with their continuations so that generic
code in the code generator will treat them as branch instruction and will be
able to apply optimization like avoiding branches when the code can falltrhough.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/798553002

Cr-Commit-Position: refs/heads/master@{#25773}
Disable generating of code cache if the debugger is loaded

BUG=440880
R=yangguo@chromium.org,dcarney@chromium.org,vogelheim@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/796823002

Cr-Commit-Position: refs/heads/master@{#25774}