[FastISel][DebugInfo] Handle dbg.value targeting allocas (#67187)

FastISel currently drops dbg.values targeting allocas. It may seem
surprising that a simple case would fail to be lowered, but dbg.values
targeting allocas are not common; we usually have dbg.declares doing
that, and those are handled by the common code between FastISel and
SelectionDAGISel.

This patch addresses the issue by querying the static alloca map from
FuncInfo. If we have a frame index for it, we create a DBG_VALUE
intrinsic from it.