[FIRRTL] Fix unnecessarily repeated calls to setType in LowerTypes.

[mikeurbach]

The previous implementation would call setType to update the module's
type attribute after each new argument was added. This constant
updating of the attribute was needed because getNumFuncArguments used
the attribute, and getNumFuncArguments was called by core MLIR logic
throughout this pass.

By updating getNumFuncArguments to just return the current number of
entry block arguments, we are able to update the type attribute once
at the end of the pass.

This is part of #615.

[mikeurbach]

CPU flame graphs seem the best way to showcase the performance gain. This was tested using the same test case as reported in #615.

Before:

After:

Note that the calls to Builder::getFunctionType and FunctionLike::setType are no longer prominent in the flame graph. On my machine, this reduced the time firtool spent in LowerTypes (according to -pass-timing) from ~60% to ~50%.

[mikeurbach]

@lattner the change in LowerTypes.cpp seems "obvious" to me, but I am less sure about the change in FIRRTLStructure.td. Do you mind taking a look? The tests pass, so I don't think anything relied on the previous behavior.

[lattner]

This looks great to me!

[lattner]

and yes, going to bb args should be faster than going through getType() which has to dig it out of an attribute.

[mikeurbach]

Thanks for the review!

[seldridge]

The nightly performance tests seem to agree that this helped. 👍

[lattner]

Great progress, but still more to do ;-)

[mikeurbach]

Yep, I will share a PR today that should clean up the remaining performance issues (spoiler: it also has to do with too many setAttr calls).