SI: Migrate bitfields to Common::BitField

[lioncash]

Migrates the SI register union bitfields over to Common::BitField for consistency with the rest of the codebase.

While we're in the same area, we can simplify a trivial conditional

[leoetlino]

Reviewed 1 of 1 files at r1, 1 of 1 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @lioncash)

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Source/Core/Core/HW/SI/SI.cpp, line 492 at r2 (raw file):

  // In and out for the 4 SI channels.
  for (u32 i = 0; i < MAX_SI_CHANNELS; ++i)

Won't this cause a sign comparison warning (because of MAX_SI_CHANNELS possibly being signed, since the underlying type isn't specified explicitly)?

[lioncash]

@leoetlino Ah, true, good catch. Amended the commit.

[leoetlino]

Reviewed 1 of 1 files at r4, 1 of 1 files at r5, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @lioncash)