Speed up HoistPlaintextOps

[j2kun]

Fixes #479

The two main problems:

1. HoistPlaintextOps used std::find_if to scan the entire generic body, only to extract a single op, leading to a quadratic scan when there were a linear number of ops to hoist. When unrolling an affine.for, each iteration gets an affine.apply op, and they can all be lifted, so it nearly always causes quadratic behavior.
2. In extractOpBeforeGeneric, the newly-created single-op generic was cloning all of the operands from the original generic op as its operands, even if it had zero secret operands. When lifting ~1k ops, this became a problem because the output of each single-op generic is added as an operand to the original generic it was lifted from, so we got into a situation like:

• start: original generic has 1 arg
• lift 1 op with 1 arg; original generic gets +1 arg
• lift 1 op with 2 args; original generic gets +1 arg
• lift 1 op with 3 args; original generic gets +1 arg
• ... (x1000)
  After that, we have a quadratic number of unused args, so we apply a quadratic number of EraseUnusedGenericArg patterns. By more finely tracking the subset of operands needed in the new generic, we can reduce this to (# ops to lift * # operands per lifted op), which is minimal. The only tricky bit is that, if the op being extracted is a loop nest, you have to walk the op's subtree to find all the generic arguments it depends on.

The test I added ran in 15s with the bug still in place, and after these fixes it runs in 0.5s.

[asraa]

Nice! 15 seconds to .5 seconds is great

[asraa]

nit: you could consider llvm::make_filter_range here