Use this to shorten error return code in the assembler.
For example, change this:
if (error = something()) {
diagnostic() << " Bad integer literal " << value;
return SPV_ERROR_INVALID_VALUE;
}
to this:
if (error = something())
return diagnostic() << " Bad integer literal " << value;
Also shorten code due to the fact that binaryEncodeU32 and
binaryCodeU64 can't fail (short of failure to expand a std::vector).
