X86: Fix conflict over ESI between base register and rep;movsl

The new solution is to not use this lowering if there are any dynamic
allocas in the current function. We know up front if there are dynamic
allocas, but we don't know if we'll need to create stack temporaries
with large alignment during lowering. Conservatively assume that we will
need such temporaries.

Reviewed By: hans

Differential Revision: http://reviews.llvm.org/D5128