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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

The crash issue should be solved, but there's still a lit failure: llvm/test/CodeGen/X86/debuginfo-locations-dce.ll
With this patch, we lack the information of `DW_AT_location` in
```
0x00000043:     DW_TAG_variable
                  DW_AT_name   ("MyVar")
                  DW_AT_decl_file      ("./test.cpp")
                  DW_AT_decl_line      (4)
                  DW_AT_type   (0x00000056 "float")
```
I know little about the pass, so I have difficulty to figure out what's wrong here. I wouldn't expect there's any functional change with this patch.

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