[SystemZ] Add optional argument to some vector string instructions

The vfee[bhf], vfene[bhf], and vistr[bhf] assembler mnemonics are
documented in the Principles of Operation to have an optional last
operand to encode arbitrary values in a mask field.

This commit adds support for those optional operands, and cleans up
the patterns to generate vector string instruction as bit.  No change
to code generation intended.

llvm-svn: 284585