Generic Pattern Matching for FP8 GEMMs in XLA
CC @reedwm. Can you resolve conflicts? a695fffc660994e892583391d69e4c1b9ce89320 has some minor conflicts.
Matches and rewrites patterns of the form

dot(unary_a1(…unary_aM(dequantize(A))…), unary_b1(…unary_bN(dequantize(B))…))

with A and B FP8 and unary_xi from a set of supported instructions into Custom Calls.
