Add DebugValue for invisible store in single_store_elim

[jaebaek]

The front-end language compiler would simply emit DebugDeclare for
a variable when it is declared, which is effective through the variable's
scope. Since DebugDeclare only maps an OpVariable to a local variable,
the information can be removed when an optimization pass uses the
loaded value of the variable. DebugValue can be used to specify the
value of a variable. For each value update or phi instruction of a variable,
we can add DebugValue to help debugger inspect the variable at any
point of the program execution.
For example,

float a = 3;
... (complicated cfg) ...
foo(a); // <-- variable inspection: debugger can find DebugValue of float a in the nearest dominant

For the code with complicated CFG e.g., for-loop, if-statement, we
need help of ssa-rewrite to analyze the effective value of each variable
in each basic block.

If the value update of the variable happens only once and it dominates
all its uses, local-single-store-elim pass conducts the same value update
with ssa-rewrite and we have to let it add DebugValue for the value assignment.

One main issue is that we have to add DebugValue only when the value
update of a variable is visible to DebugDeclare. For example,

{  // scope1
   %stack = OpVariable %ptr_int %int_3
   {  // scope2
       DebugDeclare %foo %stack    <-- local variable "foo" in high-level language source code is declared as OpVariable "%stack"
       // add DebugValue "foo = 3"
       ...
       Store %stack %int_7   <-- foo = 7, add DebugValue "foo = 7"
       ...
       // debugger can inspect the value of "foo"
    }
    Store %stack %int_11   <-- out of "scope2" i.e., scope of "foo". DO NOT add DebugValue "foo = 11"
}

However, the initalization of a variable is an exception.
For example, an argument passing of an inlined function must be done out of
the function's scope, but we must add a DebugValue for it.

// in HLSL
bar(float arg) { ... }
...
float foo = 3;
bar(foo);

// in SPIR-V
%arg = OpVariable
OpStore %arg %foo   <-- Argument passing. Out of "float arg" scope, but we must add DebugValue for "float arg"
... body of function bar(float arg) ...

This PR handles the except case in local-single-store-elim pass. It adds
DebugValue for a store that is considered as an initialization.

The same exception handling code for ssa-rewrite is done by this commit: df4198e.

[kuhar]

nit: you have a typo in the PR title / the first line of commit message, s/invisiable/invisible

[kuhar]

LGTM, but please wait for a second review before submitting.

[jaebaek]

Sure. I think @dnovillo can help.

[dnovillo]

Could you add an example in the description? I'm having a hard time picturing what the problem is.

[jaebaek]

@dnovillo I updated the description. Because of its related background e.g., DebugValue distribution, the description may be hard to understand. I will explain it to you in offline.

[dnovillo]

Thanks for the added description. It's now a bit more clear to me. One other question below.