[llvm] [SPIRV] Add support for `bfloat16` atomics via the `SPV_INTEL_16bit_atomics` extension (PR #166257)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,34 @@
+; RUN: not llc -O0 -mtriple=spirv64-unknown-unknown --spirv-ext=+SPV_KHR_bfloat16 %s -o %t.spvt 2>&1 | FileCheck %s --check-prefix=CHECK-ERROR1
+; RUN: not llc -O0 -mtriple=spirv64-unknown-unknown --spirv-ext=+SPV_EXT_shader_atomic_float_add,+SPV_KHR_bfloat16 %s -o %t.spvt 2>&1 | FileCheck %s --check-prefix=CHECK-ERROR2
+
+; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv64-unknown-unknown --spirv-ext=+SPV_EXT_shader_atomic_float_add,+SPV_INTEL_16bit_atomics,+SPV_KHR_bfloat16 %s -o - | FileCheck %s
+
+; CHECK-ERROR1: LLVM ERROR: The atomic float instruction requires the following SPIR-V extension: SPV_EXT_shader_atomic_float_add
+; CHECK-ERROR2: LLVM ERROR: The atomic bfloat16 instruction requires the following SPIR-V extension: SPV_INTEL_16bit_atomics
----------------
arsenm wrote:

If it's not available it should go through the AtomicExpand legalization 

https://github.com/llvm/llvm-project/pull/166257