IR: emitVectorReshape fails if param type is a vector whilst param value is a scalar

[chaoticbob]

Both of the assignment below causes a NULL pointer access in IRBuilder::emitVectorReshape().

sourceVectorType is null after the cast from value->getDataType().

CMD:

slangc.exe -target spirv -lang slang -D__spirv__ -emit-spirv-directly -profile vs_6_0 -entry main myshader.slang

Shader:

RWBuffer<uint>     MyRWBuffer;
RWTexture2D<int>   MyRWTexture;

void main() {
    MyRWBuffer[1].x = 15;
    MyRWTexture[uint2(2, 3)].x = 16;
}