Use accurate negative OpSMod behavior.

[squidbus]

Add option for providing OpSMod behavior accurate to the SPIR-V spec, as required by VK_KHR_maintenance8. Made it optional since it generates less clean output than just using the standard operator when this behavior is not needed, and Vulkan does not require it to be accurate without maintenance8 enabled. Probably not the most ideal implementation of this but it’s the best I could figure out how to implement in this context.

CTS results for dEQP-VK.spirv_assembly.*.maintenance8.* using MoltenVK (unsupported tests are geometry shaders):

Test run totals:
  Passed:        24/28 (85.7%)
  Failed:        0/28 (0.0%)
  Not supported: 4/28 (14.3%)
  Warnings:      0/28 (0.0%)
  Waived:        0/28 (0.0%)