This PR is to add explicit support for SPV_KHR_float_controls
(https://github.com/KhronosGroup/SPIRV-Registry/blob/main/extensions/KHR/SPV_KHR_float_controls.asciidoc).
This extension is included into SPIR-V after version 1.4, but in case of
lower versions it is to be included explicitly and OpExtension must be
present in the module with `OpExtension "SPV_KHR_float_controls"`.
This PR fixes this issue and fixes the test case
test/CodeGen/SPIRV/exec_mode_float_control_khr.ll to account for a
version lower than 1.4.
