[llvm][InstCombine] bitcast bfloat half castpair bug (#79832)

Miscompilation arises due to instruction combining of cast pairs of the
type `bitcast bfloat to half` + `<FPOp> bfloat to half` or `bitcast half
to bfloat` + `<FPOp half to bfloat`. For example `bitcast bfloat to
half`+`fpext half to double` or `bitcast bfloat to half`+`fpext bfloat
to double` respectively reduce to `fpext bfloat to double` and `fpext
half to double`. This is an incorrect conversion as it assumes the
representation of `bfloat` and `half` are equivalent due to having the
same width. As a consequence miscompilation arises.

Fixes #61984