nir: Add 64-bit integer support for conversions and bitcasts

nir: Add 64-bit integer support for conversions and bitcasts

v2 (idr): "cut them down later" => Remove ir_unop_b2u64 and
ir_unop_u642b.  Handle these with extra i2u or u2i casts just like
uint(bool) and bool(uint) conversion is done.

v3 (idr): Make the "from" type in a cast unsized.  This reduces the
number of required cast operations at the expensive slightly more
complex code.  However, this will be a dramatic improvement when other
sized integer types are added.  Suggested by Connor.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>