gallivm: Add code for rgb9e5 shared exponent format to float conversion

gallivm: Add code for rgb9e5 shared exponent format to float conversion

And use this (and the code for r11g11b10 packed float to float conversion)
in the soa texturing code (the generated code looks quite good).
Should be an order of magnitude faster probably than using the fallback
(not measured).
Tested with piglit texwrap GL_EXT_packed_float and
GL_EXT_texture_shared_exponent respectively (didn't find much else using
it).

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