Split RegisterAllocationScope out of ExpressionResult and allocate one
for each statement. This ensures that we always have an outer register
allocation scope for statement code (used in CountOperation and
RegisterExecutionResult). Also refactored the register allocator code to
move it to it's own file and rename from TemporaryRegisterScope to
BytecodeRegisterAllocator.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1587033002

Cr-Commit-Position: refs/heads/master@{#33296}