TargetMachine is not thread safe (but it really should be)

[llvmbot]

Bugzilla Link	27361
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@echristo,@joker-eph,@pcc,@rotateright

Extended Description

When doing parallel CodeGen we currently have to create a TargetMachine per-thread because TargetMachine is not thread-safe.
This is a waste, and it would be nice if it can be avoided.
Also, having an immutable TargetMachine is a preliminary step needed if we want at some point parallelize the backend.

Eric pointed out the least amount of work needed:

• Handle resetTargetOptions first (at least)
• An optional lock on getSubtarget

The subtarget itself should be fine, and if not, it should be fixed.

[echristo]

FWIW I handled a lot of resetTargetOptions, but not all of it when doing the subtarget migration work. Most of it is just figuring out how we want to propagate things through the IR.

[rotateright]

I don't know if this is actually related, but I was reminded of bug 23172.