[InstCombine] do not shrink switch conditions to illegal types (PR29009)

This patch makes shrinking switch conditions less aggressive which was introduced by:
rL274233

Note that we have 2 new bugs to track potential follow-ups that might have solved PR29009
in different ways:
https://bugs.llvm.org/show_bug.cgi?id=39569
https://bugs.llvm.org/show_bug.cgi?id=39578

Patch by:
@dendibakh (Denis Bakhvalov)

Differential Revision: https://reviews.llvm.org/D54115


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346315 91177308-0d34-0410-b5e6-96231b3b80d8