Fixes "error: conditional expression is ambiguous; 'sw::Float' can be
converted to 'RValue<sw::Float>' and vice versa".

Change-Id: I2bd21036803dc54ce212df61a7520b7276b94b2a
Reviewed-on: https://swiftshader-review.googlesource.com/19168Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>