Fix OOB in subscript indexing of col-major matrix in cbuffer #7866
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A bug in HLOperationLower code that lowers the matrix subscript operation when the matrix is in a cbuffer could cause out-of-bounds alloca indexing. The indexing creates an alloca to store a column-vector loaded from one cbuffer vector. This should have one element per row of the matrix, so it can then index along that dimension using the local array. The bug was using the number of columns when sizing the alloca instead of the number of rows. This caused it to write to out-of-bounds index when the number of rows exceeded the number of columns (like for float3x2).
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A bug in HLOperationLower code that lowers the matrix subscript operation when the matrix is in a cbuffer could cause out-of-bounds alloca indexing.
The indexing creates an alloca to store a column-vector loaded from one cbuffer vector. This should have one element per row of the matrix, so it can then index along that dimension using the local array.
The bug was using the number of columns when sizing the alloca instead of the number of rows. This caused it to write to out-of-bounds index when the number of rows exceeded the number of columns (like for float3x2).
Fixes #7865.