[X86] Relax an assertion when legalizing vector types.

[X86] Relax an assertion when legalizing vector types.

WidenVSELECTAndMask can fold (and it folds in this case) so we
get a BUILD_VECTOR of constants as mask. convertMask() seems to
work fine when the input is a vector of constants, and we still
need to call it to extend/add elements at the end. but the current
code just asserts on anything but a SETCC or AND/OR/XOR of 2xSETCC.
This change was discussed briefly with Simon Pilgrim, who also
suggests we might consider dropping this assertion in the future.

Fixes PR33715.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307508 91177308-0d34-0410-b5e6-96231b3b80d8