Move DBG_VALUE's that depend on loads to after a

load if the load is moved due to the pre register allocation ld/st
optimization pass

The issue here is that there can be a scenario where debug information
is lost because of the pre register allocation load store optimization
pass, where a load who's result describes the debug infomation for a
local variable gets moved below the load and that causes the debug
information for that load to get lost.

Example:

Before the Pre Register Allocation Load Store Pass
inst_a
%2 = ld ...
inst_b
DBG_VALUE %2, "x", ...
%3 = ld ...

After the Pass:
inst_a
inst_b
DBG_VALUE %2, "x", ...
%2 = ld ...
%3 = ld ...

The load has now been moved to after the DBG_VAL that uses its result
and the debug info for "x" has been lost. What we want is:

inst_a
inst_b
%2 = ld ...
DBG_VALUE %2, "x", ...
%3 = ld ...

Which is what this patch addresses

Differential Revision: https://reviews.llvm.org/D145168