Word64Popcnt is lowered to Word32Popcnt(low-word) + Word32Popcnt(high_word).
Since the optional Word64Popcnt operator does not exist on 32 bit platforms,
I introduced a new operator "Word64PopcntPlaceholder" which is generated
in the WasmCompiler and then lowered in the Int64Lowering.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1803453003

Cr-Commit-Position: refs/heads/master@{#34777}