[CodeGen][NFC] Fix quadratic c-t for large jump tables

[aengelke]

Deleting a basic block removes all references from jump tables, which
is O(n). When freeing a MachineFunction, all basic blocks are deleted
before the jump tables, causing O(n^2) runtime. Fix this by deallocating
the jump table first.

Test case generator:

import sys

n = int(sys.argv[1])
print("define void @f(i64 %c, ptr %p) {")
print("  switch i64 %c, label %d [")
for i in range(n):
    print(f"    i64 {i}, label %h{i}")
print(f"  ]")
for i in range(n):
    print(f'h{i}:')
    print(f'  store i64 {i*i}, ptr %p')
    print(f'  ret void')
print('d:')
print('  ret void')
print('}')

Improvement at 5000 entries:

Benchmark 1: ./llc.pre -filetype=obj -O0 <switch5k.bc
  Time (mean ± σ):      49.7 ms ±   1.0 ms
  Range (min … max):    48.0 ms …  52.1 ms    57 runs

Benchmark 2: ./llc.post -filetype=obj -O0 <switch5k.bc
  Time (mean ± σ):      39.4 ms ±   0.8 ms
  Range (min … max):    37.1 ms …  41.1 ms    72 runs

Summary
  ./llc.post -filetype=obj -O0 <switch5k.bc ran
    1.26 ± 0.04 times faster than ./llc.pre -filetype=obj -O0 <switch5k.bc

Improvement at 20000 entries:

Benchmark 1: ./llc.pre -filetype=obj -O0 <switch20k.bc
  Time (mean ± σ):     281.7 ms ±   1.0 ms
  Range (min … max):   280.2 ms … 283.0 ms    10 runs

Benchmark 2: ./llc.post -filetype=obj -O0 <switch20k.bc
  Time (mean ± σ):     123.9 ms ±   1.5 ms
  Range (min … max):   121.4 ms … 129.2 ms    23 runs

Summary
  ./llc.post -filetype=obj -O0 <switch20k.bc ran
    2.27 ± 0.03 times faster than ./llc.pre -filetype=obj -O0 <switch20k.bc

[aeubanks]

makes sense. can you put some compile time numbers in the description before and after this patch?