[llvm] [NVPTX] Improve support for {ex2,lg2}.approx (PR #120519)

[Artem Belevich via llvm-commits]

Artem-B wrote:

@Prince781 It appears that the tests are generating 32-bit PTX and it's no longer supported by recent CUDA versions.

```
[  1] ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
[  2] ; RUN: llc < %s -mcpu=sm_20 -mattr=+ptx32 | FileCheck --check-prefixes=CHECK %s [OK]
llc < third_party/llvm/llvm-project/llvm/test/CodeGen/NVPTX/f32-lg2.ll -mcpu=sm_20 -mattr=+ptx32 | third_party/llvm/llvm-project/llvm/FileCheck --allow-unused-prefixes --check-prefixes=CHECK third_party/llvm/llvm-project/llvm/test/CodeGen/NVPTX/f32-lg2.ll
[  3] ; RUN: %if ptxas %{ llc < %s -mcpu=sm_20 -mattr=+ptx32 | %ptxas-verify %} [FAIL]
 llc < third_party/llvm/llvm-project/llvm/test/CodeGen/NVPTX/f32-lg2.ll -mcpu=sm_20 -mattr=+ptx32 | third_party/gpus/cuda/_virtual_includes/_stage_runtime/third_party/gpus/cuda/bin/ptxas -arch=sm_60 -c -o /dev/null - 
ptxas warning :  64 Bit host architecture (--machine) being used mismatches with .address_size of 32 bits
ptxas fatal   :  32-Bit compilation is no longer supported
Command failed: exit status 255
```

You can reproduce it by running the tests with `LLVM_PTXAS_EXECUTABLE=/path/to/cuda-12.6.0/bin/ptxas`


https://github.com/llvm/llvm-project/pull/120519