OptimizeInstructions: Improve areConsecutiveInputsEqualAndFoldable

[kripken]

To check if we can fold A, B into A, we can just check if B has
effects we can't remove. The old code used a utility that checks if
A is also removable, but that is suboptimal, and that entire
utility is not needed.

[tlively]

LGTM if the fuzzer is happy.

[tlively]

Fuzzer is not happy! Bisected the following to this change:

;; bin/wasm-opt -all w.wast --print --optimize-instructions -S -o - --fuzz-exec
(module
 (import "fuzzing-support" "log-f32" (func $import))
 (func $test (export "test") (param $2 i32)
  (if
   (select
    (local.tee $2
     (i32.eqz
      (local.get $2)
     )
    )
    (i32.const 0)
    (i32.eqz
     (local.get $2)
    )
   )
   (then
    (call $import)
   )
  )
 )
)

[kripken]

Yikes, while this PR's changes were right, it uncovered a bug in the code it calls...

Fix in #8415