Optimize branch instructions to short form where branch distance fits in sbyte

[Copilot]

Branch instructions like Br, Brtrue, Brfalse encode a 4-byte offset by default, but have short-form variants (Br_S, Brtrue_S, Brfalse_S) using a 1-byte signed offset when the target is within ±127 bytes. Previously, a blanket replacement broke large switch statements. This PR adds proper distance-checking before selecting the shorter form.

Changes

General backward-branch optimizer (ILGeneratorTools)

• Tracks marked label positions in _ilLabelPositions (ConditionalWeakTable<ILGenerator, Dictionary<Label, int>>) — keyed by Label struct directly to avoid any hash collision ambiguity
• DmarkLabel records il.ILOffset when marking a label
• Demit(ILGenerator, OpCode, Label) uses TryGetShortFormOpCodeForBackwardBranch to automatically emit short form for backward branches (label already marked) when labelOffset - (ILOffset + 2) fits in [-128, 127]
• GetShortFormBranchOpCode maps all 13 long-form branch opcodes to their short-form counterparts
• Clears stale label position data when a pooled ILGenerator is reused (RentPooledOrNewILGenerator), preventing false short-form selection after pool reuse

Specific forward-branch optimizations (always safe, hardcoded)

Patterns where the skipped IL is provably tiny:

Site	Change	Max skipped bytes
IsFalse unary	Brfalse→Brfalse_S, Br→Br_S	3
Reference coalesce with cast	Br→Br_S	5 (Castclass)
isLiftedToNull comparison	Brfalse→Brfalse_S, Brtrue→Brtrue_S	6
Logical operator labelDone	Br→Br_S	1 (Ldc_I4_0/1)

Tests updated

Three existing opcode-assertion tests updated to expect the new short-form opcodes where the optimizer now applies (Issue498, Issue346, Issue159).