[DAG] isBitwiseNot / isConstOrConstSplat - add support for build vector undefs...

[DAG] isBitwiseNot / isConstOrConstSplat - add support for build vector undefs + truncation (PR41020)

Add (opt-in) support for implicit truncation to isConstOrConstSplat, which allows us to match truncated 'all ones' cases in isBitwiseNot.

PR41020 compares against using ISD::isBuildVectorAllOnes() instead, but that predicate silently accepts any UNDEF elements in the build vector which might not be what we want in isBitwiseNot - so I've added an opt-in 'AllowUndefs' flag that is set to false by default but will allow us to enable it on individual cases where its safe.

Differential Revision: https://reviews.llvm.org/D62783

llvm-svn: 362323