SPIRV: Add more utility functions to build some opcodes (#2398)

SPIRV: Add more utility functions to build some opcodes (#2398)

Add more builder functions to OpExecutionMode, OpExecutionModeId,
OpDecorateString, OpMemberDecorateString.

According to SPIR-V, OpExecutionMode and OpExecutionModeId could
take variable extra operands. Current implementation doesn't support
this and assumes at most 3 operands are extra operands. It is not
true. Similarly, OpDecorateString and OpMemberDecorateString could
support multiple strings either as literal strings or as string
operands. Further, OpDecorate and OpDecorateId have the same problem,
taking variable extra operands.