libclc: Fix rounding during type conversion

The rounding during type conversion uses multiple conversions, selecting
between them to try to discover if rounding occurred. This appears to
not have been tested, since it would generate code of the form:
    float convert_float_rtp(char x)
    {
      float r = convert_float(x);
      char y = convert_char(y);
      [...]
    }

which will access uninitialised data. The idea appears to have been to
have done a char -> float -> char roundtrip in order to discover the
rounding, so do this.

Discovered by inspection.

Signed-off-by: Daniel Stone <daniels@collabora.com>

Reviewed By: jvesely

Differential Revision: https://reviews.llvm.org/D81999