Changed the encoding of X86 floating point stack operations where both operands

Changed the encoding of X86 floating point stack operations where both operands
are st(0).  These can be encoded using an opcode for storing in st(0) or using
an opcode for storing in st(i), where i can also be 0.  To allow testing with
the darwin assembler and get a matching binary the opcode for storing in st(0)
is now used.  To do this the same logical trick is use from the darwin assembler
in converting things like this:

fmul %st(0), %st

into this:

fmul %st(0)

by looking for the second operand being X86::ST0 for specific floating point
mnemonics then removing the second X86::ST0 operand.  This also has the add
benefit to allow things like:

fmul %st(1), %st

that llvm-mc did not assemble.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104634 91177308-0d34-0410-b5e6-96231b3b80d8