[llvm] [NVPTX] Remove unnecessary casts (NFC) (PR #156275)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@arsenm I'm not sure if I can use `NTM.getSubtarget<NVPTXSubtarget>(F)` in other places because I only have `MF`.

The last one at line 1179 might be able to use `NTM.getSubtarget<NVPTXSubtarget>(F)` because we have `F` there.  Thoughts?


https://github.com/llvm/llvm-project/pull/156275