Don't produce unneeded dwarf info

[llvmbot]

Bugzilla Link	2188
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@asl,@jryans

Extended Description

If a function is marked nounwind then it should not
need any per-function frame moves. So emitting an
empty set of moves into the eh moves frame should be
ok. This would reduce the size. Maybe it would even
be possible to get rid of the frame altogether. These
kinds of optimizations would reduce the size of dwarf
info for functions which are marked nothrow (eg: throws())
and contain try-catch blocks.
It would be nice to reduce dwarf info to the absolute
minimum in general (it can take up quite some space in
the final executable).

[llvmbot]

I noticed that there is already some logic (use hasCalls) to
detect nounwind functions and avoid producing pointless stuff
for them. This is presumably no longer useful. What's really
needed is for someone to give this code a good think and a
good spring-cleaning.

[llvmbot]

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