For GL_EXT_vulkan_glsl_relaxed. When merging the default uniform block,
there were cases where symbols in the tree wern't updated to match the
new block structure after merging blocks together.
This change traverses the symbol tree and updates any references to the
merged block.
Avoid adding global uniform blocks to stages that don't already have it.
Otherwise multiple stages point to the same block object, and a
remapping that occurs later on will change the mapping on multiple
stages.
