[mlir][StandardToSPIRV] Fix conversion for signed remainder

Per the Vulkan's SPIR-V environment spec, "for the OpSRem and OpSMod
instructions, if either operand is negative the result is undefined."
So we cannot directly use spv.SRem/spv.SMod if either operand can be
negative. Emulate it via spv.UMod.

Because the emulation uses spv.SNegate, this commit also defines
spv.SNegate.

Differential Revision: https://reviews.llvm.org/D83679