[llvm] [AMDGPU] remove checklines for bitcast cases, only check func name (PR #139988)

[via llvm-commits]

Shoreshen wrote:

> It's an annoying large amount of checks but I don't see a reason to do this, they are generated. If you wanted to shrink the tests, we could reduce the number of tested subtargets, I guess. We could go down to 3 run lines at a minimum

Hi @arsenm , by leaving 3 target:
```
; RUN: llc -mtriple=amdgcn -mcpu=tahiti < %s | FileCheck -check-prefix=SI %s
; RUN: llc -mtriple=amdgcn -mcpu=tonga < %s | FileCheck -check-prefix=VI %s
; RUN: llc -mtriple=amdgcn -mcpu=gfx900 < %s | FileCheck -check-prefix=GFX9 %s
```
The number of auto-generated check lines for `amdgcn.bitcast.1024bit.ll` is still 178000+. Basically because there are 112 cases for complete test and each case generate few thousands lines......

https://github.com/llvm/llvm-project/pull/139988