Consecutive registers are allocated in two passes. First we "reserve"
a set of registers and these get allocated when we actually use them.
If we request for a temporary register before we use all the consecutive
registers, the earlier implementation does not gaurantee that it allocates
outside the reservation for consecutive registers. This could cause problems
for example, in call_func(a, b++, c). This cl fixes
TemporaryRegisterScope::NewRegister, to return a new temporary register
outside the reservation for consecutive registers.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1531273002

Cr-Commit-Position: refs/heads/master@{#33005}