[SystemZ] Fix parsing of inline asm registers

GPR and FPR constraints like "{r2}" and "{f2}" weren't handled correctly
because the name-to-regno mapping depends on the value type and
(because of that) the internal names in RegStrings are not the
same as the AsmName.

CC constraints like "{cc}" didn't work either because there was no
associated register class.

llvm-svn: 186148