Large increase in AMDGPU tblgen build times

[RKSimon]

Bugzilla Link	44628
Version	trunk
OS	Windows NT
Blocks	#28596
CC	@adibiagio,@arsenm

Extended Description

I've noticed -gen-dag-isel in particular seems to take a lot longer, and is by far the worst culprit in gulp debug tblgen builds

[RKSimon]

@​arsenm do you have any ideas what's caused this?

[adibiagio]

For the record: AMDGPUGenDAGISel.inc also takes ages for me to build.
Is there a way to simplify the patterns in input to gen-dag-isel?

[arsenm]

I think I did 421a40b as a result of this bug, but there's still a lot of slow things in tablegen. There are still a lot of queries on registers that repeatedly loop over every register and/or register class for each use.

Additionally, I believe the slowest parts are in target initialization code which is repeated for every individual tablegen invocation. It would be faster if we had the build invoke tablegen once with all of the backends requested at the same time, and handle the parallelization inside tablegen itself.