Don't emit Win64 pdata/xdata for "leaf" functions

[zmodem]

Bugzilla Link	30337
Resolution	FIXED
Resolved on	Sep 23, 2016 18:33
Version	trunk
OS	Windows NT
Blocks	#26673

Extended Description

From https://msdn.microsoft.com/en-us/library/ms235286.aspx:

"Leaf functions are functions that do not change any non-volatile registers. .. Leaf functions can be unwound simply by simulating a return, so pdata and xdata are not required."

Not emitting these would be good for binary size.

Note that such functions can still tail-call others, so they're not really leafs in the call-graph sense.

If we hook up a way to determine whether a function is a leaf in this sense, we can also use it to do conditional tail-calls for Win64. Such calls would generally confuse the unwinder, but in leaf functions they are fine.

[zmodem]

r282185