Finish the work in progress for handling NaN's

- gas backend: check for NaN's (parity checks) - 386, 486, 586, 686
- gas+SSE backend: check for NaN's (parity checks) - 386, 486, 586, 686
- llvm backend: not-tested
- gcc backend: already completed

- handle IR_EMITOPT_REL_DOINVERSE option in gas/gas+SSE/gas64 backends

The four scenarios passed to backend as part of the float comparison op:
- result := (a rel-op b)
- result := !(a rel-op b)
- if (a op b) then goto {falselabel}
- if !(a op b) then goto {falselabel}

test-suite: tests for float comparisons

Backends tested:
- gas: 386, 486, 586, 686, and each with SSE
- gcc: 386, 486, 586, 686, and each with SSE
- gas64: x86_64
- gcc64: x86_64
- tested all above combinations with '-fpmode fast' (Extra code
  added by the NaN checks is removed with '-fpmode fast')