[FastISel] Reduce spills around mem-intrinsic calls

FastISel generates instructions to materialize "local values" at the
top of a block, in the hope that these values could be reused within
the block.  To reduce spills and restores, FastISel treats calls as
sub-block boundaries, flushing the "local value map" at each call.

This patch treats the mem* intrinsics as if they were calls, because
at O0 generally they are calls.  Eliminating these spills/restores is
actually better for debugging (especially a "continue at this line"
command), code size, stack frame size, and maybe even performance.

Differential Revision: https://reviews.llvm.org/D90877