cmd/compile: improve error message emitted by debug info generation
Before:
unexpected at 2721:load with unexpected source op v3278unexpected at 2775:load with
unexpected source op v3281unexpected at 2249:load with unexpected source op
v3289unexpected at 2875:load with unexpected source op v3278unexpected at 2232:load
with unexpected source op v286unexpected at 2231:load with unexpected source op
v3291unexpected at 2784:load with unexpected source op v3289unexpected at 2785:load
with unexpected source op v3291
After:
debug info generation: v2721: load with unexpected source op: Phi (v3278)
debug info generation: v2775: load with unexpected source op: Phi (v3281)
debug info generation: v2249: load with unexpected source op: Phi (v3289)
debug info generation: v2875: load with unexpected source op: Phi (v3278)
debug info generation: v2232: load with unexpected source op: Phi (v286)
debug info generation: v2231: load with unexpected source op: Phi (v3291)
debug info generation: v2784: load with unexpected source op: Phi (v3289)
debug info generation: v2785: load with unexpected source op: Phi (v3291)