[clang] [PowerPC][CLANG] DMF VSX Vector float GER 2x (rank-2 update) (PR #147383)

[Maryam Moghadas via cfe-commits]

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@@ -1116,6 +1116,14 @@ UNALIASED_CUSTOM_MMA_BUILTIN(mma_xvbf16ger2, "vW512*VV",
                              "mma,paired-vector-memops")
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maryammo wrote:

Future sub target feature ; `mma,isa-future-instructions`; should be added for these builtins. 
For your reference : https://github.com/llvm/llvm-project/pull/145372#discussion_r2170066565.

Also please add these builtins in clang/test/CodeGen/PowerPC/ppc-dmf-future-builtin-err.c to test this sub target feature. 

https://github.com/llvm/llvm-project/pull/147383