[turbofan] Add SimplifiedOperator::Allocate operator.

[turbofan] Add SimplifiedOperator::Allocate operator.

This introduces a simplified allocation operator which can be used to
model inline allocations in TurboFan. It is currently used for context
allocations, but still disabled because change lowering introduces
floating allocations outside the effect chain that interfere.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1109773002

Cr-Commit-Position: refs/heads/master@{#28195}