We use variable locations such as DBG_VALUE $xmm0 as shorthand to refer
to "the low lane of $xmm0", and this is reflected in how DWARF is
interpreted too. However InstrRefBasedLDV tries to be smart and
interprets such a DBG_VALUE as a 128-bit reference. We then issue a
DW_OP_deref_size of 128 bits to the stack, which isn't permitted by
DWARF (it's larger than a pointer).
Solve this for now by not using DW_OP_deref_size if it would be illegal.
Instead we'll use DW_OP_deref, and the consumer will load the variable
type from the stack, which should be correct.
There's still a risk of imprecision when LLVM decides to use smaller or
larger value types than the source-variable type, which manifests as
too-little or too-much memory being read from the stack. However we
can't solve that without putting more type information in debug-info.
fixes#64093
cb714e74cc