[llvm] [NVPTX] Add Custom ABI (abi_preserve) support via function attributes (PR #227499)

[Durgadoss R via llvm-commits]

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+; RUN: llc < %s -mtriple=nvptx64-nvidia-cuda -mcpu=sm_80 -mattr=+ptx83 | FileCheck %s
+; RUN: %if ptxas %{ llc < %s -mtriple=nvptx64-nvidia-cuda -mcpu=sm_80 -mattr=+ptx83 | %ptxas-verify -arch=sm_80 %}
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durga4github wrote:

Can we have a test to verify the omission case where sm < 80 and ptx-ver < 83?
(with a check-not line in this file or in a separate file)

https://github.com/llvm/llvm-project/pull/227499