[MachineLICM] Let targets decide if copy instructions are cheap (#146599)
When checking whether it is profitable to hoist an instruction, the pass
may override a target's ruling because it assumes that all COPY
instructions are cheap, and that may not be the case for all
micro-architectures (especially for when copying between different
register classes).
On AArch64 there's 0% difference in performance in LLVM's test-suite
with this change. Additionally, very few tests were affected which shows
how it is not so useful to keep it.
x86 performance is slightly better (but maybe that's just noise) for an
A/B comparison consisting of five iterations on LLVM's test suite (Ryzen
5950X on Ubuntu):
```
$ ./utils/compare.py build-a/results* vs build-b/results* --lhs-name base --rhs-name patch --absolute-diff
Tests: 3341
Metric: exec_time
Program exec_time
base patch diff
LoopVector...meChecks4PointersDBeforeA/1000 824613.68 825394.06 780.38
LoopVector...timeChecks4PointersDBeforeA/32 18763.60 19486.02 722.42
LCALS/Subs...test:BM_MAT_X_MAT_LAMBDA/44217 37109.92 37572.52 462.60
LoopVector...ntimeChecks4PointersDAfterA/32 14211.35 14562.14 350.79
LoopVector...timeChecks4PointersDEqualsA/32 14221.44 14562.85 341.40
LoopVector...intersAllDisjointIncreasing/32 14222.73 14562.20 339.47
LoopVector...intersAllDisjointDecreasing/32 14223.85 14563.17 339.32
LoopVector...nLoopFrom_uint32_t_To_uint8_t_ 739.60 807.45 67.86
harris/har...est:BENCHMARK_HARRIS/2048/2048 15953.77 15998.94 45.17
LoopVector...nLoopFrom_uint8_t_To_uint16_t_ 301.94 331.21 29.27
LCALS/Subs...Raw.test:BM_DISC_ORD_RAW/44217 616.35 637.13 20.78
LCALS/Subs...Raw.test:BM_MAT_X_MAT_RAW/5001 3814.95 3833.70 18.75
LCALS/Subs...Raw.test:BM_HYDRO_2D_RAW/44217 812.98 830.64 17.66
LCALS/Subs...test:BM_IMP_HYDRO_2D_RAW/44217 811.26 828.13 16.87
ImageProce...ENCHMARK_BILATERAL_FILTER/64/4 714.77 726.23 11.46
exec_time
l/r base patch diff
count 3341.000000 3341.000000 3341.000000
mean 903.866450 899.732349 -4.134101
std 20635.900959 20565.289417 115.346928
min 0.000000 0.000000 -3380.455787
25% 0.000000 0.000000 0.000000
50% 0.000000 0.000000 0.000000
75% 1.806500 1.836397 0.000100
max 824613.680801 825394.062500 780.381699
```
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