[SPIRV] Fix failing assertion in SPIRVAsmPrinter (#166909)
With `+SPV_KHR_float_controls2` and when there is a non-int
`OpConstantNull` we
would call `MI.getOperand(1).getImm()` when `MI` was not an `OpTypeInt`
(the
associated test has an `OpTypeArray` zeroinitialized).
Under this conditions an assertion is triggered.
This patch adds the missing condition.
