Clarified the direction rounding occurs in uitofp and sitofp is unspecified

[mswadling]

Summary

LLVM does not specify which direction to round ints when converting to floating point numbers. On x86, the direction depends on the status register. By specifying this in the LangRef it ensures no assumptions are made.

[scanon]

I'm uncomfortable with this change, as I believe that it makes the llvm conversions incompatible with IEEE-754 conversion (which round according to the dynamic rounding mode unless they're in a (language defined) block with static rounding attribute).

I would much prefer that we say something like "On targets with dynamic rounding modes, [su]itofp rounds according to the dynamic mode. On targets without dynamic rounding, [su]itofp uses default rounding."

[chandlerc]

What scanon said.

docs/LangRef.rst

 The '``uitofp``' instruction interprets its operand as an unsigned
 integer quantity and converts it to the corresponding floating point
 value. If the value cannot fit in the floating point value, the results
-are undefined.
+are undefined. The direction the number is rounded is unspecified.
 
 Example:
 """"""""

 The '``sitofp``' instruction interprets its operand as a signed integer
 quantity and converts it to the corresponding floating point value. If
 the value cannot fit in the floating point value, the results are
-undefined.
+undefined. The direction the number is rounded is unspecified.
 
 Example:
 """"""""