[XLA] Fix HloElementTypeConverter's handling of bitcast convert.

Previously, if you asked HloElementTypeConverter to e.g. convert BF16 to F32,
it would convert a BF16 input to bitcast-convert to F32.

This don't work, because the output of the bitcast-convert still has width 16
bits, and the input/output widths must match.

The right thing is to ignore bitcast-convert operations for the purposes of
this pass.

PiperOrigin-RevId: 232554328