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[WebAssembly] Fixed byval args missing DWARF DW_AT_LOCATION

Authored by aardappel on Jan 5 2021, 4:58 PM.

Description

[WebAssembly] Fixed byval args missing DWARF DW_AT_LOCATION

A struct in C passed by value did not get debug information. Such values are currently
lowered to a Wasm local even in -O0 (not to an alloca like on other archs), which becomes
a Target Index operand (TI_LOCAL). The DWARF writing code was not emitting locations
in for TI's specifically if the location is a single range (not a list).

In addition, the ExplicitLocals pass which removes the ARGUMENT pseudo instructions did
not update the associated DBG_VALUEs, and couldn't even find these values since the code
assumed such instructions are adjacent, which is not the case here.

Also fixed asm printing of TIs needed by a test.

Differential Revision: https://reviews.llvm.org/D94140