Simplify dbg.value handling in SDISel with early returns

No functional change other than improving dbgs logging accuracy on
constant dbg values. Previously we would add things like "i32 42" as
debug values, and then log that we were dropping the debug info, which
is silly.

Delete some dead code that was checking for static allocas. This
remained after r207165, but served no purpose. Currently, static alloca
dbg.values are always sent through the DanglingDebugInfoMap, and are
usually made valid the first time the alloca is used.

llvm-svn: 302267