Fixes issues raised on: https://reviews.llvm.org/D133926
The patch above enabled using variadic-form debug values to represent single-location, non-stack-value debug values, and a further patch made all DBG_INSTR_REFs use variadic form. Not all code paths were updated correctly to handle the new syntax however, with entry values in still expecting an expression that begins exactly DW_OP_LLVM_entry_value, 1.
A function already exists to select non-variadic-like expressions; this patch adds an extra function to cheaply simplify such cases to non-variadic form, which we use prior to any entry-value processing to put DBG_INSTR_REFs and DBG_VALUEs down the same code path. We also use it for a few DIExpression functions that check for whether the first element(s) of a DIExpression match a particular pattern, so that they will return the same result for DIExpression(DW_OP_LLVM_arg, 0, <ops>) as for DIExpression(<ops>).