This commit fixes a crash in the compiler when lowering a groupshared
variable with a multi-dimensional array type. The root cause of the bug
was that we had a nested gep expression that could not be merged into a
single gep because of an intervening addrspacecast.
The `MultiDimArrayToOneDimArray` pass flattens the multi-dimension
global variables to a single dimension. It relies on the `MergeGepUse`
function to flatten any nested geps into a single gep that fully
dereferences a scalar element.
The fix is to modify the `MergeGepUse` function to look through
addrspacecast instructions when trying to merge geps. We can now merge
geps like
gep(addrspacecast(gep(p0, gep_args0)) to p1*, gep_args1)
into
addrspacecast(gep(p0, gep_args0+gep_args1) to p1*)
We also added a call to `removeDeadConstantUsers` before flattening
multi-dimension globals because we can have some dead constants hanging
around after merging geps and these constants should be ignored by the
flattening pass.
481090a822