Implemented preserves_flags on powerpc by making it do
nothing. This prevents having two different ways to mark
`cr0` as clobbered. clang and gcc alias `cr0` to `cc`.
The gcc inline documentation does not state what this does
on powerpc* targets, but inspection of the source shows
it is equivalent to condition register field `cr0`, so it
should not be added.
Where supported, VSX is a 64x128b register set which encompasses
both the floating point and vector registers.
In the type tests, xvsqrtdp is used as it is the only two-argument
vsx opcode supported by all targets on llvm. If you need to copy
a vsx register, the preferred way is "xxlor xt, xa, xa".
Despite that the `fflags` register (representing floating point
exception flags) is stated as a flag register in the reference, it's not
in the default clobber list of the RISC-V inline assembly and it would
be better to fix it.
