TBR: i#511 drreg: keep aflags in eax where possible

Commit log for first patchset:
---------------
i#511 drreg: keep aflags in eax where possible

Improves drreg's aflags preservation on x86 by keeping it in eax initially,
instead of blindly spilling to TLS.  Without this optimization, the samples
doing their own increment insertion produced much better code than using
drx_insert_counter_increment() with drreg.
---------------

[bruening, 2017-01-30 05:43:23]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/00d39c73c9d23b5ee76889c4e9157f4...

Final commit log: 
---------------
i#511 drreg: keep aflags in eax where possible

Improves drreg's aflags preservation on x86 by keeping it in eax initially,
instead of blindly spilling to TLS.  Without this optimization, the samples
doing their own increment insertion produced much better code than using
drx_insert_counter_increment() with drreg.

Review-URL: https://codereview.appspot.com/315480043
---------------