Fix getting operand without checking opcode.

Fixes https://github.com/KhronosGhttps://github.com/KhronosGroup/SPIRV-Tools/issues/1559roup/SPIRV-Tools/issues/1559.

There is an load of an operand of an instruction that was suppose to be
only for the OpCompositeExtract case.  However, an error caused it to
be loaded for every opcode, even those that do not have an operand in
that position.

We fix up that bug, and a couple other things noticed that the same
time.