[Frontend] add fuse-elementwise-ops pass

[erick-xanadu]

Context: stablehlo lowering creates many elementwise operations. It is possible to fuse them and thus reduce code size. For example:

import pennylane as qml
import catalyst
import jax

MATSIZE=128 #scale with me

@catalyst.qjit(keep_intermediate=True)
#@jax.jit
def func(a, b):
    return jax.scipy.linalg.expm(1j*a)

a = jax.numpy.outer(jax.numpy.array([2]*MATSIZE), jax.numpy.array([2]*MATSIZE))
b = jax.numpy.outer(jax.numpy.array([3]*MATSIZE), jax.numpy.array([3]*MATSIZE))

print(func(a,b))

Produces a stablehlo of 1176 lines after stablehlo lowering and 14039 lines in LLVM.
After this change that goes down to 759 and 9936 respectively.
I also expect this to possibly reduce execution time, but I haven't benchmarked it yet.

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

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[paul0403]

Is it possible to do this a bit earlier in the pipeline so the execution time of more passes can benefit from the code size reduction?