[llvm] [PowerPC] extend smaller splats into bigger splats (PR #141282)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

I reproduced the ICE encountered in the AIX build (https://lab.llvm.org/buildbot/#/builders/64/builds/3890/steps/5/logs/stdio) by compiling an AIX-preprocessed version of `compiler-rt/lib/builtins/muldc3.c` with `-target powerpc-ibm-aix -O3`.

The ICE goes away after reverting this PR.

https://github.com/llvm/llvm-project/pull/141282