[llvm] [LiveDebugValues] Use getNumSupportedRegs to reduce compile time, NFCI (PR #114944)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

Verified with valgrind with below command
`vg-in-place --tool=callgrind clang -DNDEBUG  -O3 -fomit-frame-pointer -DNDEBUG -g   -w -Werror=date-time -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DSQLITE_OMIT_LOAD_EXTENSION=1 -DSQLITE_THREADSAFE=0 -I. -MD -MT shell.c.o -MF shell.c.o.d -o shell.c.o -c MultiSource/Applications/sqlite3/shell.c`

| Patches  | Instructions | Ratio |
| ------------- | ------------- | ------------- |
| Without #113532  | 1790650933  | - |
| With  #113532 | 1793289982  | +0.15% |
| With  both | 1790138180  | -0.03% |

https://github.com/llvm/llvm-project/pull/114944