PPC: Limit unbound label tracking to branch references.

PPC: Limit unbound label tracking to branch references.

Labels which are not associated with branches (e.g. labels which
record the location of the embedded constant pool or jump tables)
should not be tracked for the purpose of trampoline generation.

This also improves management of the high water mark in the buffer
which triggers trampoline generation such that it is reset whenever
the number of tracked branches drops to zero.

These changes should help minimize unnecessary trampoline and
(subsequent) slow branch generation.

R=dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1237213002

Cr-Commit-Position: refs/heads/master@{#29659}