mem: Fix initial value problem with MemChecker
mem: Fix initial value problem with MemChecker

In highly loaded cases, reads might actually overlap with writes to the
initial memory state. The mem checker needs to detect such cases and
permit the read reading either from the writes (what it is doing now) or
read from the initial, unknown value.

This patch adds this logic.

TUD-OS/gem5-dtu,rallylee/gem5,rallylee/gem5,briancoutinho0905/2dsampling,gedare/gem5,KuroeKurose/gem5,Weil0ng/gem5,zlfben/gem5,Weil0ng/gem5,kaiyuanl/gem5,SanchayanMaity/gem5,rallylee/gem5,kaiyuanl/gem5,powerjg/gem5-ci-test,SanchayanMaity/gem5,kaiyuanl/gem5,markoshorro/gem5,HwisooSo/gemV-update,Weil0ng/gem5,gem5/gem5,KuroeKurose/gem5,zlfben/gem5,markoshorro/gem5,gem5/gem5,cancro7/gem5,Weil0ng/gem5,briancoutinho0905/2dsampling,cancro7/gem5,markoshorro/gem5,gedare/gem5,gem5/gem5,briancoutinho0905/2dsampling,KuroeKurose/gem5,cancro7/gem5,rjschof/gem5,sobercoder/gem5,powerjg/gem5-ci-test,SanchayanMaity/gem5,qizenguf/MLC-STT,aclifton/cpeg853-gem5,powerjg/gem5-ci-test,austinharris/gem5-riscv,HwisooSo/gemV-update,gem5/gem5,austinharris/gem5-riscv,zlfben/gem5,gem5/gem5,rjschof/gem5,TUD-OS/gem5-dtu,rallylee/gem5,markoshorro/gem5,zlfben/gem5,TUD-OS/gem5-dtu,cancro7/gem5,zlfben/gem5,KuroeKurose/gem5,zlfben/gem5,briancoutinho0905/2dsampling,HwisooSo/gemV-update,powerjg/gem5-ci-test,gedare/gem5,gedare/gem5,KuroeKurose/gem5,SanchayanMaity/gem5,markoshorro/gem5,Weil0ng/gem5,sobercoder/gem5,markoshorro/gem5,HwisooSo/gemV-update,sobercoder/gem5,gedare/gem5,Weil0ng/gem5,gem5/gem5,austinharris/gem5-riscv,HwisooSo/gemV-update,HwisooSo/gemV-update,powerjg/gem5-ci-test,rallylee/gem5,qizenguf/MLC-STT,austinharris/gem5-riscv,SanchayanMaity/gem5,sobercoder/gem5,austinharris/gem5-riscv,Weil0ng/gem5,cancro7/gem5,cancro7/gem5,qizenguf/MLC-STT,sobercoder/gem5,austinharris/gem5-riscv,briancoutinho0905/2dsampling,qizenguf/MLC-STT,TUD-OS/gem5-dtu,kaiyuanl/gem5,rallylee/gem5,aclifton/cpeg853-gem5,powerjg/gem5-ci-test,cancro7/gem5,sobercoder/gem5,qizenguf/MLC-STT,aclifton/cpeg853-gem5,briancoutinho0905/2dsampling,rallylee/gem5,aclifton/cpeg853-gem5,TUD-OS/gem5-dtu,qizenguf/MLC-STT,gedare/gem5,HwisooSo/gemV-update,TUD-OS/gem5-dtu,rjschof/gem5,rjschof/gem5,kaiyuanl/gem5,briancoutinho0905/2dsampling,aclifton/cpeg853-gem5,gedare/gem5,qizenguf/MLC-STT,gem5/gem5,TUD-OS/gem5-dtu,KuroeKurose/gem5,KuroeKurose/gem5,aclifton/cpeg853-gem5,kaiyuanl/gem5,sobercoder/gem5,aclifton/cpeg853-gem5,kaiyuanl/gem5,SanchayanMaity/gem5,austinharris/gem5-riscv,rjschof/gem5,markoshorro/gem5,SanchayanMaity/gem5,rjschof/gem5,rjschof/gem5,powerjg/gem5-ci-test,zlfben/gem5