Fix an assert in collectUses when handling unknown index offsets.

[atrick]

AccessPathDefUseTraversal accumulates the offsets that it has seen on
the def-use walk while visiting index_addr and casts.

This is quite tricky because either the AccessPath being matched or
the def-use chain could contain unknown offsets. We need to correctly
classify all cases as either an exact, inner, or overlapping match.

For SIL like this

%91 = integer_literal $Builtin.Int64, 0
%113 = builtin "truncOrBitCast_Int64_Word"(%91 : $Builtin.Int64) : $Builtin.Word
%115 = index_addr %114 : $*MyStruct, %113 : $Builtin.Word
%119 = struct_element_addr %115 : $*MyStruct, #MyStruct.klass

We have Path: (@unknown,#0)

There are two issues

(1) When AccessPathDefUseTraversal::checkAndUpdateOffset visits the
struct_element_addr, it fails to pop the unknown offset from the
expected path but continues trying to handle the struct_element_addr
and hits an assert.

That's probably as simple as fixing

< if (pathOffset > 0) {

to be

< if (pathOffset != 0) {

(2) If the builtin "truncOrBitCast_Int64_Word" comes from a literal,
we should not treat it as an unknown offset (given that the literal is
within the size of a word).

We should change the test case above to be a truly unknown offset to
test issue #1. Then add the test case as-is to
accesspath_uses_ossa.sil to test that the constant offset is recognized.

Those can be separate fixes.

[meg-gupta]

Thanks for the fix! I merged this fix with tests in #38757. PR to get non-unknown access paths in the presence of builtins like truncOrBitCast will be a follow up.