Avoid read past end of operands vector for EOpFrexp

When emitting SPIR-V code for frexp, avoid access
beyond the end of the operands vector.  When constructing
the OpExtInst, construct a new arguments vector instead of
modifying the existing operands vector.  In the case of OpFrexp,
well need that last operand later on to generate the store.

Fixes https://github.com/KhronosGroup/glslang/issues/110

Change-Id: Ibc380fadf5e600ac491932e9ecef7afe2d72fd7f