Improve optimization of select between same values

[tlively]

We previously optimized selects between the same value only when the ifTrue and ifFalse values had identical structure and no side effects and the condition could be dropped or reordered with the value.

Make this more robust by using the areConsecutiveInputs... utilities to check whether the arms have equal values and whether they are foldable. The old code avoided introducing blocks to keep code size down, but the blocks it would introduce are not named and will be elided in the output, so introducing blocks is not a problem. The old code also avoided introducing a scratch local, but if a scratch local is necessary, it is because the arms have some side effects that would prevent reordering. In that case it is more likely to be beneficial to execute only one arm, so go ahead and use scratch locals in the case where the second arm can be folded away.

[kripken]

Fuzz bug:

$ bin/wasm-opt w.wasm -all -tnh  --optimize-instructions
wasm-opt: src/wasm-builder.h:1537: static wasm::Index wasm::Builder::addVar(wasm::Function*, wasm::Name, wasm::Type): Assertion `type.isConcrete()' failed.

(module
 (rec
  (type $9 (func))
  (type $14 (func (result f32)))
  (type $29 (func (param i64) (result v128)))
  (type $26 (func (param v128) (result v128)))
  (type $11 (func (result f64)))
 )
 (func $1 (type $11) (result f64)
  (unreachable)
 )
 (func $2 (type $26) (param $0 v128) (result v128)
  (unreachable)
 )
 (func $5 (type $29) (param $0 i64) (result v128)
  (unreachable)
 )
 (func $8 (type $14) (result f32)
  (unreachable)
 )
 (func $30 (type $9)
  (nop)
 )
 (func $80 (type $9)
  (unreachable)
  (unreachable)
  (block
   (drop
    (select (result (ref none))
     (ref.as_non_null
      (ref.null none)
     )
     (ref.as_non_null
      (ref.null none)
     )
     (i32.wrap_i64
      (i64.trunc_sat_f64_u
       (call $1)
      )
     )
    )
   )
   (unreachable)
   (unreachable)
  )
 )
)

[tlively]

Oops, I fixed that this morning but apparently never uploaded a PR. Here it is now: #8817