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Decades ago Intel decided that the bsr (Bit Scan Reverse) instruction
should have undefined results if its argument is zero. This probably
makes the instruction harder to implement and it definitely makes it
more difficult to use.

In SkCLZ_portable it requires a check for a zero argument, but despite
that check /analyze still warns that _BitScanReverse might fail
(because it doesn't know what can cause failures). Because this warning
occurs in a frequently included header file it ends up being very noisy,
accounting for ~30% of all warnings (before deduplication).

Suppressing this useless warning will make the raw results easier to
look through.

Review URL: https://codereview.chromium.org/872673007