[SPIR-V] Remove switch G_ICMP+G_BRCOND+G_BR before ISel

IRTranslator lowers switches to [G_SUB] + G_ICMP + G_BRCOND + G_BR
sequences. Since values and destination MBBs are included in the
spv_switch intrinsics, the sequences are not needed for ISel.

Before this commit, the information decoded by these sequences were
added to spv_switch intrinsics in SPIRVPreLegalizer and the sequences
were kept until SPIRVModuleAnalysis where they were marked skipped for
emission.

After this commit, the [G_SUB] + G_ICMP + G_BRCOND + G_BR sequences
and MBBs containing only these MIs are erased in SPIRVPreLegalizer.

Differential Revision: https://reviews.llvm.org/D146923