Added some extra constructors that simply truncate from a type
which has a higher number of bits. The new constructors are:
- Byte from UInt
- Byte from UShort
- SByte from Int
- SByte from Short
- UShort from UInt

Also added an implementation of the RoundUInt function using the
UInt from Float constructor, which had to be fixed since it was
using createFPToSI instead of createFPToUI.

Change-Id: Ie7ee21ef20fbb8133b9f7c74afa1fec9e6c51957
Reviewed-on: https://swiftshader-review.googlesource.com/4300Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>