Implemented Float <-> Half conversion. The conversion is copied from
Half.cpp and is the same as the C++ code, converted to Reactor code.
The conversion is NOT optimized. Optimizing this code path will be
done later, this cl only cares about correctness.

Passes all conversion tests to and from half float in the following
patterns:

[dEQP-VK.api.copy_and_blit.core.blit_image]
.all_formats.color.[supported format].r16_sfloat.*
.all_formats.color.[supported format].r16g16_sfloat.*
.all_formats.color.[supported format].r16g16b16a16_sfloat.*
.all_formats.color.r16_sfloat.*
.all_formats.color.r16g16_sfloat.*
.all_formats.color.r16g16b16a16_sfloat.*

Bug b/119620767

Change-Id: Icd725dcd2ddd8c54e6657a2c3210da270210a149
Reviewed-on: https://swiftshader-review.googlesource.com/c/23329Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>