This pass tries to fold f2f16 conversion into alu instructions.
This will be useful to help reduce the number of instructions once
mesa starts supporting precision lowering.  For example:

  add.f r0.w, r0.w, c0.x
  cov.f32f16 hr2.x, r0.w

to

  add.f hr2.x, r0.w, c0.x

Additionally this pass also tries to fold f2f16 conversion into load_input
instruction:

  bary.f r0.x, 3, r0.w
  cov.f32f16 hr0.x, r0.x

to

  bary.f hr1.x, 3, r0.x

v2: Edit to not fold OPC_MAX_F and OPC_MIN_F, since that's not valid.

v3: Add OPC_ABSNEG_F to the blacklist as well.

v4: Don't remove dead cov instructions, DCE will do that later; don't
iterate through sources when a cov only has one; remove special
handling of IR3_REG_ARRAY and IR3_REG_RELATIV.

v5: Handle folding into u32.u32 movs of floats correctly, don't bail
out on IR3_REG_RELATIV or IR3_REG_ARRAY movs.

Part-of: <mesa!3822>