When we added the new MachineRepresentation::kTaggedSigned and
MachineRepresentation::kTaggedPointer, we didn't extend the logic
for memory operand covering, and so for map checks and other
comparisons with fields we'd always need an additional register.
This fixes that and does reduce register pressure in some cases.

R=jarin@chromium.org
BUG=v8:5267,v8:5270

Review-Url: https://codereview.chromium.org/2354863003
Cr-Commit-Position: refs/heads/master@{#39575}