[CIR] Add support for ExtVectorBoolType element assignment, comparisons, and logical NOT

[lanza]

Stack from ghstack (oldest at bottom):

• [CIR] Implement constant expression member pointer casts #2017
• [CIR] Add support for GNU ifunc attribute #2012
• [CIR] Add support for polymorphic typeid with vtable lookup #2007
• -> [CIR] Add support for ExtVectorBoolType element assignment, comparisons, and logical NOT #2004
• [CIR] Fix RTTI mangling assertion with qualified types #2002
• [CIR] Add support for typeid l-value emission #2001
• [CIR] Fix assertion failure in CIRGenRecordLayout for Empty Base Optimization #1999
• [CIR] Add support for ExtVector Bool Type #1998
• [CIR] Implement APValue emission for member function pointers #2016
• [CIR] Implement static variable initialization with guarded init #2015
• [CIR] Implement cast expression classes as l-values #2011
• [CIR] Implement non-virtual thunk generation for multiple inheritance #2006

This commit implements three previously missing operations for ExtVectorBoolType:

1. Element assignment (v[i] = value): Converts the packed integer representation
   to a padded vector, performs the element insertion, and converts back to
   integer form while preserving padding bits.

2. Vector comparisons (==, !=, etc.): Extracts actual elements from padded
   storage using shufflevector, performs element-wise comparison, then pads
   the result back to storage size with undef for padding bits.

3. Logical NOT (!v): Implemented as element-wise comparison with zero vector,
   following the same extract-operate-pad pattern as other comparisons.

All operations follow the ExtVectorBoolType storage model where N bool elements
are stored in an integer padded to at least 8 bits (1 byte). The implementation:

• Bitcasts the integer storage to

  vector (P = padded storage size)

• Uses shufflevector to extract (actual elements)
• Performs the operation on actual elements
• Uses shufflevector to pad result back to

  (undef for padding)

• Bitcasts back to integer storage

This approach ensures padding bits remain undefined/unchanged and mirrors how
LLVM CodeGen handles these operations.

Test Plan:

• Added comprehensive tests for element assignment, comparisons, and logical NOT
• Tests verify both CIR output and LLVM lowering
• All existing CIR CodeGen tests pass (395 tests, 380 passed, 14 unsupported, 1 expected fail)
• Specific vector tests pass: vector.cpp, vectype.cpp, vectype-issized.c, vectype-ext.cpp

The implementation closely follows CodeGen patterns in CGExprScalar.cpp and
CGExpr.cpp, maintaining consistency with the original implementation approach.