[WebAssembly][NFC] RelaxedBinary tablegen multiclass for relaxed SIMD

Refactor the tablegen definitions for relaxed SIMD min/max instructions to use a
shared RelaxedBinary multiclass modeled on the existing SIMDBinary multiclass. A
future commit will add further instruction definitions that use RelaxedBinary.

Also rename the SIMD_RELAXED_CONVERT multiclass to RelaxedConvert to better fit
existing naming conventions.

Reviewed By: aheejin

Differential Revision: https://reviews.llvm.org/D127157