Fix 5fecd146

Word32ReverseBytes and Word64ReverseBytes are mandatory
so their definition is removed from list of optional operators
in InstructionSelector::SupportedMachineOperatorFlags

Change-Id: I997fca1da6fcb69d1d6190a5ff68f3ff1f201b45
Reviewed-on: https://chromium-review.googlesource.com/1175129
Commit-Queue: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Commit-Queue: Sreten Kovacevic <skovacevic@wavecomp.com>
Reviewed-by: Sreten Kovacevic <skovacevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#55137}