[msan] Do not store origin for clean values.

Instead of unconditionally storing origin with every application store,
only do this when the shadow of the stored value is != 0.

This change also delays instrumentation of stores until after the walk over
function's instructions, because adding new basic blocks confuses InstVisitor.

We only keep 1 origin value per 4 bytes of application memory. This change
fixes the bug when a store of a single clean byte wiped the origin for the
whole 4-byte area.

Since stores of uninitialized values are relatively uncommon, this change
improves performance of track-origins mode by 5% median and by up to 47% on
specs.

llvm-svn: 169490