TBR: i#1795 drreg: move xl8 slowpath clearing to inlined instru

Commit log for first patchset:
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i#1795 drreg: move xl8 slowpath clearing to inlined instru

Moves the clearing of translation address sharing when the slowpath is
entered (due to clobbering the register) from C code, where we need to
know the TLS slot (which is not drreg-friendly) and we need a 2-step return
path, to the inlined instrumentation at the return-from-slowpath point.
This increases the code cache size slightly but should enable removal of
the 2-step return and makes moving to drreg easier.
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[bruening, 2017-01-13 05:23:21]

Committed as
https://github.com/DynamoRIO/drmemory/commit/31250dfb76127ac96070fe2def38309f...

Final commit log: 
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i#1795 drreg: move xl8 slowpath clearing to inlined instru

Moves the clearing of translation address sharing when the slowpath is
entered (due to clobbering the register) from C code, where we need to
know the TLS slot (which is not drreg-friendly) and we need a 2-step return
path, to the inlined instrumentation at the return-from-slowpath point.
This increases the code cache size slightly but should enable removal of
the 2-step return and makes moving to drreg easier.

Review-URL: https://codereview.appspot.com/311520043
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