[SystemZ] Make better use of VLLEZ

This patch fixes two deficiencies in current code that recognizes
the VLLEZ idiom:

- For the floating-point versions, we have ISel patterns that match
  on a bitconvert as the top node.  In more complex cases, that
  bitconvert may already have been merged into something else.
  Fix the patterns to match the inner nodes instead.

- For the 64-bit integer versions, depending on the surrounding code,
  we may get either a DAG tree based on JOIN_DWORDS or one based on
  INSERT_VECTOR_ELT.  Use a PatFrags to simply match both variants.

llvm-svn: 349749