Handle ftz flag correctly

[vosen]

Expected outcome:
All instructions with .ftz modifiers correctly flush subnormals (or not, if the flag is unset). Even in the modules with mixed ftz-on/ftz-off instructions.

Comments:

• Most of the time a module will either have all instructions ftz-on or all instructions ftz-off. Implementation should make use of that fact and visit all instructions if we are in such case. Consult with with LLVM, I think AMDGPU LLVM supports ftz attribute function-wide
• If it's mixed ftz mode then best course of action is to set ftz off and flush per-argument. ptx_parser and its visitors (visit/visit_mut/visit_map) should be extended to report ftz information. Changes to generate_instruction_type! will also be necessary to expose per-argument ftz information.
• llvm.canonicalize can probably do the flushing, needs checking