gallivm: bring back optimized but incorrect float to smallfloat optimizations

gallivm: bring back optimized but incorrect float to smallfloat optimizations

Conceptually the same as previously done in float_to_half.
Should cut down number of instructions from 14 to 10 or so, but
will promote some NaNs to Infs, so it's disabled.
It gets a bit tricky though handling all the cases correctly...
Passes basic tests either way (though there are no tests testing special
cases, but some manual tests injecting them seemed promising).

v2: style and comment fixes suggested by Jose

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>