ssa bootstrap problem on x86 (cmpstrsi_1 pattern)

[Joern Rennecke]

> The only reason clobbers appear this early in the compiler is that GCC
> generates machine-specific code very early.  There is no operation

That is not true.  When gcc synthesizes a move that is wider than the
widest one available in the machine description, it first clobbers the
destination, then does a series of moves into SUBREGs of the destination.