Fix uint64->{float, double} conversion to do rounding correctly in 32-bit.

The implementation in LegalizeIntegerTypes to handle this as 
sint64->float + appropriate power of 2 is subject to double rounding,
considered incorrect by numerics people.  Use this implementation only
when it is safe.  This leads to using library calls in some cases
that produced inline code before, but it's correct now.
(EVTToAPFloatSemantics belongs somewhere else, any suggestions?)

Add a correctly rounding (though not particularly fast) conversion
that uses X87 80-bit computations for x86-32.

7885399, 5901940.  This shows up in gcc.c-torture/execute/ieee/rbug.c
in the gcc testsuite on some platforms.

llvm-svn: 103883