This is according to the expected KHR_vulkan_glsl without floating point.
So, floating-point spec-const operations no longer work, and that's
reflected in the tests.
Move SpecConstantOpModeGuard from makeSpvConstantFromConstSubTree() to
visitbinary() and visitunary(). Checking if the visiting node is a spec
constants, if so, turn on the SpecConstantOpMode, otherwise, stay in the
normal mode.