[PPC] Do not emit extswsli in 32BIT mode when using -mcpu=pwr9

It looks like in some circumstances when compiling with `-mcpu=pwr9` we create an EXTSWSLI node when which causes llc to fail. No such error occurs in pwr8 or lower.

This occurs in 32BIT AIX and BE Linux. the cause seems to be that the default return in combineSHL is to create an EXTSWSLI node.  Adding a check for whether we are in PPC64 before that fixes the issue.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D87046