Fix serialization of whens with empty blocks

[jackkoenig]

Also get rid of whitespace-only lines that were emitted after every when block.

This fixes 2 bugs (1 functional, 1 just a little ugly):

1. Whens with empty Blocks (ie. Block(Seq())) were emitting with just an empty line, this fixes it to have a skip
2. The line after a when was blank (except it had the same number of spaces as the indenting in the when block)

This is best illustrated with some simple Chisel

class Example extends RawModule {
  val a, b = IO(Input(UInt(8.W)))
  val c, d = IO(Output(UInt(8.W)))

  d := b
  when (a === b) {
  }
  c := a
}

Emitted with spaces replaced by _ to visualize whitespace

Current master/1.5.x (Chisel 3.5-SNAPSHOT Scastie):

__module_Example_:
____input_a_:_UInt<8>
____input_b_:_UInt<8>
____output_c_:_UInt<8>
____output_d_:_UInt<8>

____d_<=_b
____node__T_=_eq(a,_b)
____
____when__T_:
______
____c_<=_a

This PR:

__module_Example_:
____input_a_:_UInt<8>
____input_b_:_UInt<8>
____output_c_:_UInt<8>
____output_d_:_UInt<8>

____d_<=_b
____node__T_=_eq(a,_b)
____when__T_:
______skip
____c_<=_a

Which matches the behavior of 3.5.4 (3.5.4 Scastie).

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• [NA] Did you update the FIRRTL spec to include every new feature/behavior?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• [NA] Did you add text to be included in the Release Notes for this change?

Fix for unreleased bug so no need for release notes.

Type of Improvement

• bug fix

API Impact

No impact

Backend Code Generation Impact

This fixes emission of .fir

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[seldridge]

This looks fine.

Would it be even better to just not emit the when at all in this case as opposed to an empty when?

[jackkoenig]

Would it be even better to just not emit the when at all in this case as opposed to an empty when?

Perhaps, but this PR is just fixing a bug, restoring the old behavior.

It's also arguably good to have the empty when for debugging your Chisel by looking at the .fir, but anyway, beyond the scope of this PR.