Investigate the heuristics for creating DummyDef in LSRA

[kunalspathak]

Today, if there is a CSE candidate has definition in the middle of deeply nested loop, we end up adding DummyDef refpositions in blocks that doesn't have its uses and that leads to spilling/reloading such variables without need. This happens because of the way we choose the predecessor of such blocks and we should revisit to see if we can also accommodate the variable location where it is defined instead of just checking the block's flow. Relevant discussion in #70271 (comment)

category:design
theme:register-allocator
skill-level:expert
cost:small
impact:medium

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Issue Details

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Today, if there is a CSE candidate has definition in the middle of deeply nested loop, we end up adding DummyDef refpositions in blocks that doesn't have its uses and that leads to spilling/reloading such variables without need. This happens because of the way we choose the predecessor of such blocks and we should revisit to see if we can also accommodate the variable location where it is defined instead of just checking the block's flow. Relevant discussion in #70271 (comment)

Author:	kunalspathak
Assignees:	kunalspathak
Labels:	area-CodeGen-coreclr
Milestone:	8.0.0