Handle element_size_in_bits in constant folding.
Handle element_size_in_bits in constant folding.

Constant folding runs HloEvaluator, which creates Literals. Before, if element_size_in_bits was nonzero in a constant folded op, an error would occur since Literals do not support element_size_in_bits and so CHECKed it was zero in the constructor. Now Literal will silently set element_size_in_bits to zero in the constructor. 

Because the newly created constant-folded constant op derives its Shape from the constant-folded literal, now HloConstantFolding explicitly sets element_size_in_bits on the newly created constant op since the Literal will always have element_size_in_bits set to zero.

This will be needed to support int4 in arbitrary ops on CPUs/GPUs.
